Can I donate Troops to enemy players?
No. Troops can only be donated to teammates, and up to a limited amount per 24 hours.
Is there a way to "steal" Troops from other players?
No, there is not.
What happens to troops when you change teams?
Troops in Atlas will stay where they are when a player changes Teams. While the player retains ownership of the Troops, their team affiliation will change along with that of the player. This means: if those troops are still sitting on the previous Team's castle, they can be attacked. It's highly recommended for anyone who intends to change teams to move all their Troops to a neutral territory before doing so.
On a certain time of the day, the game checks all players on Atlas if they still have access to it. Once found, it transfers all the troops from the primarchs they own back to their Barracks. If you are unable to see any attacks happening on your Battle Ledger, rest assured that they were safely transferred back to your Barracks, but the primarch itself is freed since all troops has been placed back.