If we can avoid micromanagement hell, a mechanism to allow captured armor, aircraft, and ships especially should be included in HOI4. The Germans equipped many formations with captured equipment to make up for manufacturing shortfalls, and passed down a lot of it to their allies when it was obsolescent to their ToE. Czech 35t and 38t tanks made up a lot of the front line armored units in Poland, France, and early Russia. The 38t was so good Germany kept the production line going until 1942 and when the tank itself was obsolescent it continued to manufacture the chassis for Marder and Hetzer SP guns. Captured French S-35 and R-35 tanks were used by Germany and it's allies. Germany also used captured Russian artillery in abundance, particularly the 76mm M1939 USV and the 76mm M1942 ZIS-3, both of which were excellent multi-purpose weapons. Everyone should know the lengths that the UK went to prevent the French navy from being turned over to the Germans. However, both Germany and Italy did manage to incorporate French naval units into their fleets, and could have gotten many more powerful naval units with better diplomacy and/or effort. And of course every country commandeered opponents transport vehicles whenever able.