For home campaigns, do as you wish! For canon gaming, though, the only PrC's permitted are in the setting guide (more are in the upcoming revised guide), or check the Round Table Rules forums.
However - post a flavor text of what the PrC would be in T13K, and the PrC it is otherwise the same as, and we'll discuss it (basically to ensure it fits properly, and if not we'll offer advice on making it fit).
T13K doesn't use any PrC's except as listed in the setting guide, though, as we felt generic PrCs were not the direction we wanted to go.
Again - only the flavor text need to change, making it T13K-specific instead of generic.