You can use prepared mustard. Depending on the quantity of crab cake mixture your recipe makes, I would suggest starting with about 1/4 teaspoon Dijon or plain yellow mustard to start. You don't want the mustard to jump out at you. If you need to add more, do so to your taste.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!