I wish I would have gotten the DCT. But in your case OP, I think best to trade for a 6MT M3 and then start the mods. Probebly save you alot of money and time/effert and I doubt it would ever realy be the same.
I can't imagine it being cost effective to convert. Probebly easier to convert from a 6MT to a DCT (just guessin) because you would have to add a clutch pedal and all the supporting linkage/brackets, etc. But then again a DCT tranny is like $7,000 last I saw and the modding would probebly be another few thousand minimum, plus software update to run it.
eithe way, not worth the cost. Good luck!