[BirdOPrey5]

It can be done without a mod with usergroups and promotions.

Make a new usergroup with the same permissions as your Registered Usergroups is now.

Then limit the permissions of registered users to only be able to post new threads and only in an introduction forum, with no permission to post elsewhere on the site.

Then setup a usergroup promotions to promote users with 1 post to the new usergroup (that had the same permissions as "Registered Users" used to.)

Since they are only allowed to post in the introduction forum and only allowed to make a new thread (not reply to anyone else's) the only post they can make would be an introduction of some sort.

The only thing to remember is the promotion isn't effective immediately, I believe on most forums it's a cron job that runs every 15 minutes.