Submit application form upon subscription

Hi, I'd like registered users to fill a specific application form upon subscription upgrade, so that administrators can evaluate the request befor approval
The form might be created with a non CB plugin, in example I'm using breezing forms
Is it possible?

Or, as an alterntative, would it be possible to implement something similar when a user ask to join a group in groupJive?

No, there's no functionality for that in CBSubs. Best I can suggest is configure your plans to only display if directly accessed by URL (see Workflows tab) when used for registration/upgrade. Next create your own plan display (e.g. a Joomla article) that first links them to your custom form (e.g. using RSForms), which once submitted takes them to the CBSubs plans page (or I guess you can provide that link after reviewing their form).

There's no functionality for that in GJ either and would likely need to do the same thing or try to intercept their join request using CB Auto Actions and a redirect action on the gj_onBeforeJoinGroup trigger. You could store in a field whether they're allowed to join or not and condition against that in your auto action to determine if they need to be redirected to your custom form or not.