How can I sell additional Custom Capabilities to existing members?
If you want to sell new Custom Capabilities to existing members, you can generate a Capability (Buy Now) form that has been configured to add one or more Custom Capabilities to the members' account once they have completed payment for the new Capabilities. See: WordPress Dashboard → s2Member → PayPal Pro-Forms → Custom Capability (Buy Now) Forms
If you only want existing members to have access to this form, be sure to place the shortcode for the form on a Post or Page that is restricted to existing members. Or, you can wrap your Pro-Form inside Simple Shortcode Conditionals.