For a while there was PENN 66 and B-75, but these were both taken off the market because strings made from them often broke without any warning.
Right now dacron (B-50 or B-500) is your only choice. You can add strands to the string to reduce stretch and improve durability a little, but you'll loose some performance.
I don't know if you could safely turn a non-FF bow into a FF bow. The string grooves would have to be cut at the proper angle, and tip overlays applied, but I couldn't say if it would work out.
Chad