How can i attach the attachment-ID to the user profile image?

Generally speaking you can add the attachment ID to the user’s profile using the User Meta API, then display the image using the show_user_profile and edit_user_profile hooks.

Adding the image to the user listing can be done using the manage_users_columns and manage_users_custom_column filters, which Robert hue covered in this answer: https://wordpress.stackexchange.com/a/160423/2807