Try looking here:
http://www.ngemu.com/forums/showthread.php?t=42985
http://forums.emulator-zone.com/showthread.php?t=76
A long time ago, I tried to do the same: use NRage plugin, and saved games from a GB emulator. Couldn't do it because I couldn't run P64 correctly... anyway, you should be able to make it work if you played a Pokemon game from an emulator and saved your game in a .sav file and then load it with the NRage plugin...
It's been quite a while since I attemped to do that, so anyone else could confirm this?