If you mean just renaming the frigate class name, that's a simple change to the string file. If you mean you want to name individual frigates names like the TDN Colossus... well that's an entirely different matter. I know of no mod that's done that, and as capitalships and starbases are the only entities that can have their own names, the best you could do is make all the frigates in the game capital ships but still mostly act like Frigates. That would be a lot of work though, and I say mostly because there are a couple things you can't change (like them costing a capital ship point) that would have to be worked around.
