It would be a cool concept, although maybe make it not as effective in enemy hands, since most guns besides generic howitzers have ammo types that aren't readily accessible in foreign countries. Tanks can have pretty simple turret replacements for the most part, but capturing for example an m1 garand, which (I believe) has a specific clip since it's semi auto would be a problem since your country would have to start manufacturing clips for the handful of guns.
The British had a good idea. While America was still neutral, the NRA held a drive to collect rifles to send to the allies, and even sent over caches of British made world war 1 rifles. The only problem was these rifles were specifically made to use American ammo, which was scarce in Britain, so they distributed the majority of the donated rifles to the homeguard troops(who had mainly been armed and trained with pitchforks and the like, and were happy to have any gun not sent to the real troops) consisting of 4F's and old men(still fearing invasion at the time).
Maybe add a feature on the division pop up we saw in WWW where you can decide if a unit gets the newest equipment or uses whatever is available, and adds a check box to equip a unit with seized equipment(be good in combination with your old infantry weapons once you upgrade, especially for MPs)