Onagers as siege equipment only?

[Julio-Claudian]

How might I go about making onagers only buildable when laying siege, like siege towers etc.? I just find them quite irritating on open battlefields.
Is it even possible?

[Augustusng]

I've wanted to do something like this since Rome 2 was released. Unfortunately, I don't think it's possible.

[Julio-Claudian]

I've wanted to do something like this since Rome 2 was released. Unfortunately, I don't think it's possible.

Grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr

Thanks for the reply