The conversion's pretty easy really, just so long as you have a good machinist doing the job.
The RS shaft will simply press out of the lower clamp, then make a new shaft using the NSR250 dimensions. Remember though that it'll have to press fit into the RS lower clamp so this bit should mimic the RS shaft in it's dimensions.
Or, if you use an original NSR shaft, remove it from the NSR lower clamp as explained earlier then press and weld a bush on the bottom of the shaft to mimic the RS shaft and press that puppy into the RS lower clamp.
Done it yet?