You can transfer it, just.. copy paste the world.. And, it's not just a matter of changing ID's.. if that were the case, you could just simply copy paste configs.. The thing is... The world has the blocks stored by ID, so you will have to put the tekkit ID's in FTB.. which means, you will have to do so EVERY UPDATE. So.. yeah, that. I think you could install Id Solver, then make a dump, and the modify in every version the config of the ID solver only.. But don't know if it will work! I think is worth at try at least! Let me know if it works
BTW: Welcome to Feed The Beast !!! I hope you have fun!