You can use Free Mode to create custom battles, where you can place up to 5 different teams each with their own units. It's recommended that you control one of the teams, rather than having the teams fight each other.

To do this you build the unit/s from any of the various faction build bars, and deploy the unit/s to any where on the map; moving them around the map if need be. Next you select which team the unit/s will be controlled by, I suggest Team 5 to start with as the enemy team.

After that you can either repeat the above ending with a different team or begin the battle, with the units you just deployed.

If you wish to change the previous teams units, whether adding new units or moving existing ones around the map, your going to have to assign your own units you want to control to Team 1. They will come back under your control once you begin the battle or remove them from Team 1.