Could you not imagine the Proud Boys being allowed to serve as a unit if Canada seized Montana and Idaho, and the national guard needed resources? (With the assumption that the US had a GDP equivalent to Ukraine's.)
Keep in mind Ukraine is a very poor country being attacked by a much larger, richer, and more powerful country. This would be more comparable to Mexico deputizing the cartels if the US decided to invade.
If you have a limited headcount, a country invading you, and a violent bunch of nationalists who want to fight the invaders, you're going to turn them down?