The list of volunteers for Germany from non-occupied, non-allied countries or from POW's is extremely short.
There were a lot from occupied territories and allies, but those are represented ingame as the manpower you get from conquest and expeditionary forces.
Swedes: about 200 in the Waffen SS, so hardly noticeable. More fought in the Finnish army, one batallion's worth.
Switserland: no more then a 1000, so 1 MP
Spain: at least 45.000 volunteers, all from the 'Blue Legion'
All neighbouring countries.
Italy send some too, but they are allied to Germany so they can send expeditionary forces.
There were a lot of foreign men that fought for Germany during the war (estimated at 2 million!), but most were from occupied territories.
As for the winter war (from
wikepedia);
1,010 Danes, 8,700 Swedes, 725 Norwegians, 372 Ingrians, 346 Finnish expatriates, 366 Hungarians and 210 volunteers of other nationalities
12000, of which 10400 from their neighbouring countries.
An ingame system to represent volunteers would be quite difficult IMO, but perhaps worth it. A little help for the Fins during the winter war, some help for Germany.
I'd limit it to neighbouring neutral countries that are very close to your own ideology.
Perhaps add a cost for accepting those volunteers? They most likely wouldn't be able to speak the language, so special treatment and training would be necessairy.
Here's an idea: if you have high enough relations and are close enough in ideology, you could get a 'licence build' for a brigade along with some manpower. That way, you can name the brigades in the event
