Gluing in lenses
Lenses can be glued in to a mask. This is often done for scuba masks, and I don't think there would be any reason you couldn't do this with a Seal mask. I would inquire at your local scuba shops and see what opticians in town offer that. They usually send them somewhere -- you can send them directly, if you look in scuba magazines, but I think you would have better customer service from a local optician, and they can measure the mask you want to use more accurately than you can.