I am going to say doubtful... The reason I say this is that the fzr600 engine can fit into the 400 frame, and I would thus expect that the 400 -> 250 conversion isn't possible. I would also think that the 250 frame is just too light to hold that engine. Having said that nothing is impossible... but given that the weight of the 250 is 145 kg, and the 400 is 165 - I would expect the majority of weight difference to be in the engine size and not the actual frame - not sure what other benefit you would be looking at getting... The people over at http://p072.ezboard.com/bexupbrotherhood may be able to help (ask in the fzr 400 board or general discussion section - there are some EXTREMELY knowledgable people there. The 250 board is mostly made up of the same people as here, and it receives very little traffic anymore)
