I actually have a similar question, but different too. I actually own a NWN Diamond DVD version, and thus have my own keys, however the critical rebuild is harder to find nowadays, would my keys work for installing the GOG version along with kingmaker?
Yes, I've done this with a couple of existing CD keys from disc versions of the game I already owned. When you send away for the GOG CD keys, they will send you instructions along with an .ini file. You just edit the CD keys in the .ini file to the ones you want to use, save it and then follow the rest of the instructions as written.