editing a php forum script

First off I'm not sure if your aloud to alter the php script of a forum, but on my site I have it set up so that you have to sign up and in my forum you have to make a userneame/password also can I not make it so my visitors use thier main username in the forum? any help would be great- please explain in english php is not my strong point Thanks!

You sure can! All you have to do is remove one of the databases (either your forums or your own registration system) and combine them to create one table that both of the applications can interface to. Can't explain it any better than that because you'll have to dig in the code and do some editing. Shouldn't be too hard.