If you swap engine and transmission from the rwd to the awd one I think you should swap the front subrame too so it will be lighter, the front knuckles, the driveshaft, differential and so on... suspension, subframe (so engine), differential should be plug and play and yes, then you can code the car to rwd, but it seems there are some tricky things to consider:
"The transmission mounting points are different, there's a spot where the tunnel is clearanced to make room for the transfer case. That's not insurmountable, you basically will have to fabricate a transmission mount that uses the holes that the xDrive transmission mount does.
The ABS system is also different, which is trickier than it appears on the surface. You'll have to harvest the lines and the abs pump out of the donor."
from this topic
https://www.e90post.com/forums/showthread.php?t=1987220
The steering rack has a different ratio but if you swap the subframe you can swap the rack
https://www.e90post.com/forums/showthread.php?t=1603231
do an extensive search on the forum, but if you have some time and fab skills it should be doable