Currently, you can put your military units back in your inventory by following any of these:
-After you built a unit in your military buildings, press the X button at the bottom right screen where the Battle Map/Campaign button is located. Only works for undeployed units.
-If you already deployed your units, the next best thing to do is to sell them in your market. Ask a friend to buy them from you and buy the items back from their markets. Only works for "unlocked" units, meaning players can only buy units that they can build. This will only work on Limited Edition units if both parties have purchased the contracts.