[Opserty]

Not sure if there is a modification for this, but if you don't find one then:

1. Create a new usergroup with the same permissions as the "Registered Users" usergroup, (we'll call this say 'Full Users' for this example)
2. Move all users currently in the "Registered Users" Usergroup to the "Full Users" Usergroup
3. Alter the permissions for the "Registered Users" Usergroup so that they only have permission to create threads & posts (and possibly not reply to other threads) in your introduction forum.
4. Create a Usergroup Promotion to promote users from "Registered Users" to "Full Users" when they have created a post. (Set Posts to 1)

Info you might need:

Adding or Editing a Usergroup
Usergroup Promotions
Usergroups and Permissions