Can Crohn's disease cause stomach ulcers?

Yes. Crohns disease can affect any portion of the GI tract from the mouth to the rectum.
Yes. Crohn's disease of the stomach is a less common location, but it is certainly possible. Furthermore steroids, which are common treatments for crohns can CAUSE ulcers as well. So can ibuprofen, which many people use to treat pain.
Yes. Crohn's disease can affect the entire GI tract and has been known to cause both stomach ulcers and oral ulcers.