How to integrate with 3rd-party member registration

The site is developed by other developer using Membership Pro. To support extended profile fields, Profile Builder Pro plugin is used. And the developer link the registration with Membership Pro payment process somehow.

Because we need to implement Stripe as primary payment gateway, switching to Protected Content seems to be a natural choice. Without basic developer manual, it is like walking in the dark.

Make it simple, are there any way I can hack the membership registration step 1 (user profile), to add more (many more) custom user profile fields. Or can I use existing approach, use other (Profile Builder Pro) plugin for members registration, and link to the payment process offered by Protected Content?