For starters, this is one of the more difficult areas to mod correctly. The game can be very particular on how you make edits and the NDF's have an unintuitive structure.
I have not dug around enough to give a definitive yes or no, but I suspect adding a new, blank nation is possible. I'm doubtful you can implement it as a neutral though, as the game is designed around REDFOR and BLUFOR, and you definitely cannot create new units. You do have the option of repurposing an existing nation and all of its units if you prefer a simpler method, though you are still looking at an involved project.
If you want to do your own experimenting, I can imagine creating a new nation may be a similar process as adding new deck specializations, with maybe several more steps. (A variety of locations must be able to cross reference each other). I'd begin my exploration here.viewtopic.php?p=953636#p953636