You'd have to pry my cold dead fingers away from my winSPWWII CD edition widescreen
I don't normally play winSPMBT, great game, but not my area of interest (not sure I even have a patched version of the free game installed).
If you want players to be able to choose from a variety of nations, there are two theatres that immediately come to mind: Eastern Front with players as German or Axis minor powers; Western Front with players as any of a number of Allied nations.
For the Eastern Front there were also SS units comprised primarily of foriegn volunteers (and sometimes conscripts), modeling them is quite possible from the German OOB.
If you absolutely must have multinational core forces with winSPWWII you could simply provide players with a map and condition specifications (buy points, visibility, length, engagement type, date, sides), the players would then create the first scenario using the map buying only the HQ unit for the opposing player but otherwise buying their core force. They save the scenario and send it to you where you can then setup the opfor, save scenario and use it to create a custom campaign.
This would allow the player to buy allied/captured units from other nations for their core force. It would also allow them to more easily customize formations to model specific TOEs. Of course, the honor system would have to be invoked to avoid anyone modifying unit data for nefarious purposes.
The other option, though more work for you, is just to have the players send you their desired core force TOEs.
Brian