Is there such thing as ..."Step Mother -in-law"?

My Ma (stepmother) fits the bill quite nicely.

My wife and she get along very well.

But my wife never met my mother, since she died when I was a teenager.

It depends on how you look at her. Like my husband don't consider his step mom his mother because he doesn't look at her as a Mother. Age has something to do with it too. The only time i say step mother in law is if I am talking to certain people trying to tell who she is.

Do what is comfortable to You!

You can refer to her either as your father-in-law's wife or your step mother-in-law. I would call her by her first name, however, when around people who know her relationship already.