I married US citizen and am staying outside US.
Is it a good idea to state in essays(probably optional one) that I'm going to apply for a green card once admitted?
It is only recommended to use the optional essay to talk about situations that can significantly affect your candidacy. While the potential of getting permanent residence during your time in b-school may have some bearing (i.e. putting you in the domestic pool, and have broader job opportunity), it does not significantly change your candidacy. I would advise against writing an option essay for this. I may come off as having a poor sense of judgement.