Yep Sam, that's exactly what I did with the seat on my BA. I added a couple of inches to the Gunfighter/Speedmaster style seat. I also narrowed it a bit by cutting off the sides by a few inches also, as you can maybe see from these pics of my bike.
It really is pretty easy to do. Just remove the seat cover by prying off the staples, then add the foam using 3M 90 spray adhesive, cut the foam using an electric carving knife, and once you have it cut the way you want it and smoothed out using a wide and somewhat flat piece of heated metal(I used an old gardening trowel), then apply a large 1/4 inch thick piece of foam across the entire seat and use the spray adhesive to apply that.
I found a local auto interior shop that then made a new cover for it, and all for just 75 bucks.
