Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

 

Yes, you can, for taking another wife is not obligatory in Islam, it is Mubah meaning neutral and up to each person’s choices and circumstances in life.

 

So if conditions in the Nikah contract were neither Haram nor preventing an obligation, only then we say you cannot include it in a Nikah contract.

 

But if she conditioned things like to finish her study or not to move her out from the town she lives in or that you do not marry one with her and you agree to that, yes you can include it.

 

Almighty Allah knows best.