How to add a script that after a specific event changes that faction's unit and building roster?

[trueman11]

Specifically, I want to add some buildings and units (which I separately will create) to a barbarian faction, after it destroys a faction with a different culture (roman, eastern, etc.).

For example, if I play as Gaul, after destroying the Julii I get new buildings and units.

(Edit: It seems I posted this in a wrong forum section..sorry)