I've heard of a few folks converting 2001-2004 models. The big difference is that the 5EAT talks to the engine computer a lot more than the old 4EAT did. This will present extra challenges.
You may need to make up a dummy TCU to respond to the ECU. Or maybe you could have some new custom ECU programming done- as weird as it sounds that kind of thing is getting more common.
You might be able to retrofit an older ECU along with the older throttle body and older management sensors but this may involve changing a lot of the wiring harness and would make the car unable to pass emissions in some states.
I won't say impossible, but it would be a big pile of work.
You actually got me curious about this... it looks like the European Spec B had the 3.0R engine mated to a 6MT. It might be cheapest to go over there, buy one and then import it back to the USA. Definitely the most reliable result anyway...