How much do you want to extend it? its just a couple self tapping bolts holding it on to the frame. Get hunks of steel or aluminum the thickness you need, drill a couple holes in them, and use longer bolts to space it down. I would think an inch would be enough? 1" x 1" square stock should work. I doubt anything is commercially available. I can make them and ship if you can't do it or get it done locally. The spacer needs to solidly support the bump stop, washers on the ends will result in damaged parts.
Burdar's idea has merit too, but I'd have to look to see if there is room to mount something on the axle where it is needed.