Tom,
There is also another (more direct) solution to your problem.
Simply set as Player1 the US and then buy whatever "captured" equipment.
Then you may add to your force selection any required units from your Belgian Allies
cheers,
Pyros
p.s In this later approach the only visual difference should be that your HQ unit will have US nationality... I also fear that in the first approach (even if you change the player's 1 flag to US) all captured equipment will still be bought as Belgian units (I will check it later).