Add new panel inside parent panel in WP Customize?

Not possible as of now.

You can however achieve it by extending WP_Customize_Panels class.

How To extend WP_Customize_Control
How do I remove a pre-existing customizer setting?
Theme customizer – settings order
Custom Image section in Customizer
Get entered value of customiser field for live preview
How to remove the Theme Customization Button from the dashboard and themes options page?
Is it possible ( or advisable) to allow open access to the new theme customizer for potential clients?
Add a dropdown to theme customizer
Theme Customizer : how to create multiple-level panel
Allow Shortcode in Theme Customizer
Theme Customizer – Nested Sections?
Customizer Active Callback not working
Can I create customizer setting that can handle plugin shortcode?
Editing the custom background CSS
How to execute conditional script when on new customize.php (Theme Customize) screen
Is the theme customizer slowing down my site?
Trigger Customizer saving process with Javascript only
Adding customizer styles with wp_add_inline_style
WordPress Customizer Typography: How to load just the unique Google Fonts?
WordPress Customize — Move “menus” options / field to another section
Can we add more than one control under a setting in WP theme customizer?
Widgets panel not displaying in the Theme Customizer
Is it possible to display the admin bar while in the Theme Customizer?
How do I get a parent theme modification from a child theme?
Changing the entire control choices using wp.customize with JavaScript
How to Change CSS Variable value in Theme Customizer Live Preview
How to remove menus section from WordPress theme customizer
Get background color for Live Preview with Theme Customization API?
Checking if there is an Image inserted – if not don’t display anything
“Display Site Title and Tagline” checkbox not working?
How to set page template on front using starter content?
Default header image does not display
WordPress Customizer Control with React
Responsive Images – Generating multiple images from Theme Customizer control upload?
Modify Javascript Configuration Options for Theme Customizer Colour Picker
Modify the Additional CSS section (adding a disclaimer)
How to change the customizer´s sidebar width?
Theme Customizer – Choose where widget area appears, to let users organise widgets
wordpress custom theme import and export options for sample data [closed]
How to hook CSS file according to theme selection in the customizer section
How i can get widgets areas working in customizer?
How Can I Create a List of Values to Be Iterated Through via WordPress Customization API?
Change setting name in Customizer and keep the data
after_setup_theme, Global Variable and Theme Customizer
Custom Script Section Only Echoes Text
How to disable automatic colors in the Twenty Twenty theme?
WordPress not registering Theme Customization Section
How can i add edit shortcut icon in wordpress customizer without using selective refresh?
WP Customize refresh problem
Input ordering in wordpress theme customizer
Unable to pass variable on theme cusomizer add_settings()
How to add field customizable by the theme editor to your theme?
Get a setting value conditionally in the Customizer api
How to add new args data in $wp_customize->add_setting?
WP_Customize_Color_Control omitting # symbol
Hide header texts (Title & Tagline) in custom-header.php
get_option in header.php not returning value from customizer
Internal error 500 accessing Theme > Customize
Customizer: Unique identifier that distinguishes which image upload control is uploading an image
Using an if statement in the theme customizer page
Theme-wide custom post template
Edit footer via customizer
How do i make a custom section’s setting display the default value in Customizer?
How to add styles set by user in the customizer
Disabling Customizer Selective Refresh shortcut icons for selected controls
JavaScript stops working on selectively refreshed sections one inside the other
Folder name for a pro version of a WordPress.org theme?
Display theme layouts [closed]
“Add A Widget” button in the Customizer
Can I add multiple arrays within add_theme_support( ‘colors’ )?
Customizer: save setting/control content to post/page
Q: How to pull data from custom table to populate zustomizer setting/control select options
Using WordPress’ Theme Customizer to select page templates that update layout in the preview
Dynamically generated wordpress customizer options?
Is accessing theme and using customizer GPL distribution?
How to disable wordpress from overload my stylesheet styles with customizer styles
The ‘default’ attribute from $wp_customize->add_setting() method not in db?
Override customizer values on a per-menu base
How to modify default controls in WordPress theme customizer
How to enqueue javascript for WP Customize options sidebar?
Is it possible to visually group items on the theme customizer?
Theme Customizer not loading JS for live preview
Customizer – loading settings/controls/sections/panels based on a id/page id
Change WordPress header color using customizer
Default Text not showing from customizer
Is there any way to register custom background images like header images?
How to transfer one domain to another domain without losing theme options (customizer)?
Why in the static pages of my theme are showing these information?
Theme Customizer not displaying saved values in wp_head (CSS)
Is there a WordPress boolean for “theme_customizer_active()”?
Can I make a Customizer dropdown-pages list include private pages?
How to allow certain PHP functions when using sanitize_callback in the word press customizer
Customizer Color Control Not Working
where is real values of variables?
How to allow users to create their own website within my domain? [closed]
Doing context-aware previews in the WordPress Customizer
How to set Post meta-box defaults based on the choices made by user in Customizer?
Alignment Problem [closed]
Avada Child Theme Development
Questions from a new WP Dev [closed]

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