If you're meaning what I think you're meaning then the short answer is no, not without destroying the basics of fs2 gameplay by altering the .fs2 reading in the source code (or something similar). (Yes that was the short answer
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The fighters and weapons available to the player are determined by the mission files themselves.
From what I gather - you want to fly new ships in the main campaign missions? Well aside from ripping open the mission files and adding the new fighters to the "team-loadout", it can't be done without ruining the game engine.
The issue with the source code is that you'd be redifining how the engine interprets the mission files - and by doing that not only would the tech-room allow everything so would normal campaigns (i.e. a mission designed for the player flying a Herc II now has "Super-duper-kill'em-all" fighter in it!)
Sorry to disapoint!
If I'm making totally the wrong assumptions about what you mean feel free to correct me!
(Edit was to make this make more sense)