The NAD receivers use NEC1 IR protocol, device 135.124. Some UEI remotes have a setup code for NAD receivers, and it is Audio 0320. Unfortunately, the 8820N does not carry this setup code, although the older 8820B00 does. I do not see any setup code that could be modified to work either.

I'm not sure what you mean by learning mode. If you mean using the 975 command to learn the IR signals from the OEM remote, then the 8820N should learn these signals easily. If you mean the setup code search feature, then that's not going to work.