How can I give siege weapons to barbaric factions?

[Vlad-Tudor]

Or for that matter add some units from one faction to another. I tried, but went face on into a wall of text for each unit. I did not changed that, since it looked as complicated as able to crash the game.

[Ludens]

Check out Epistolary Richard's guide on adding new units (https://forums.totalwar.org/vb/showthread.php?51750-Tutorial-Adding-new-units-for-beginners). And remember to make a back-up of the relevant files before editing them.