Not tried , but true
Place a 3/4" thick piece of stock between the rear of the square chisel and the fence and rotate the chisel until it's flat against the strip, still holding it against the fence as well. Then tighten the chisel in the holder. Use a piece whose thickness that is the same distance from the edge to the fence as you want your mortises and you'll be all set to go.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)