I don't think so. Reading the MekTek forums I've stumbled on some information on how the game handle's the mechs. It seems that for the single player mode, the free market has pre-designed slots for each mech and getting a mech from the MekPak (not Mech Pack) means buying all the mechs of one type from the market, then selling one back (or not), and then buying the same type again, only for it to be a mech from the MekPak.
As I said, weird.