On the function to register a new custom post type can you set this CPT as Submenu to a exist menu item. Use the param show_in_menu
A example:
register_post_type(
'issue',
apply_filters( 'wpit_register_issue_post_type',
array(
'labels' => $issue_labels,
'rewrite' => $issue_rewrite,
'supports' => $issue_supports,
'taxonomies' => $issue_taxonomies,
'menu_position' => 5,
'public' => TRUE,
'show_ui' => TRUE,
'show_in_menu' => 'edit.php',
'can_export' => TRUE,
'capability_type' => 'post',
'query_var' => TRUE
)
)
);
You find the string for the items on mouse over, like upload.php
on the item Media.