adding motors an ok, or not?

Well, I’m not very “with it” when it comes to the technical side of robotics, but today at our meeting, the guys who have been there for a while seem to be in agreeance that we can only use the motors that FIRST supplies us with and then we can add gear boxes or whatnot if we want. I was under the impression that we were restricted to what FIRST gives us in our kit of parts when it came to motors.
Can anyone clear this up for me?

See rules <R20> and <R39> in Section 5 (“The Robot”) of the manual.