Do you mean purchasing fighters? Well, in theory, you could do that. That would be very high level FREDing however.
Basically, you'd need a campaign persistent variable and a "hub mission" with a decision mechanism. You'd write the player's amount of money to the variable, adding to it the value of each money-making action performed, then when players triggers the purchase of a fighter, subtract it's price from the variable and allow the newly purchased ship using "allow-ship" SEXP. That's the basic idea, while it sounds fairly simple, the implementation would be rather complex.