Balancing aside, off the top of my head, it should be possible to add an Interceptor to the manufacturing requirements, but I'm not sure how the game would handle that since aircraft aren't quite like other items. How would the game decide which fighter to remove from its corresponding hangar, if you have more than one Interceptor?
And there's also the fact there might be some transformation taking place when one such craft is delivered to your base. You buy a generic Interceptor, but when it arrives it becomes more unique and given an individual name (i.e. INTERCEPTOR-1). I'm not sure the game would recognize an individualized aircraft as valid manufacturing material.
A third problem might arise from the game's need to have a free hangar to manufacture a new fighter, even if in this case the "upgrade" would be replacing one instead of building one from scratch. The game probably wouldn't know the difference. It's likely that, for instance, if you had two hangars and two Interceptors, you'd need a vacant third hangar to be able to improve a fighter. And that's not very elegant.
