In fact the option exists but rarely works...It is a counter-effect of the "too easy bribe system" of RTW...

The amount of the bribe is determined automatically by the AI according to the valor of the army you're bribing, the level of your diplomat and supposedly the amount of cash in your treasury...

Even with cheats, and level 6 diplomats I haven't been able to bribe even a 4- units army...

I posted sometime ago about how impossible it was to bribe and hopefully, it seems to be moddable.

see this thread :

https://forums.totalwar.org/vb/showthread.php?t=81751