You could do it, but it is better if you use the entire system which includes the drain. While thinset might hold the bottom edge to the fiberglass, that would be the weak link. The fiberglass pans tend to be pretty flimsy. If you do use one, make sure to fully support it underneath, or you will be looking at stress fractures in short order. You might want to order a tube of the Kerdifix, a urethane adhesive designed to attach Kerdi where things might flex and thinset won't be reliable. If you can use their preformed pan, then I'd stay away from the fiberglass pan and use it instead. It depends on the size and shape of the shower and where the drain is located. You can cut their pan, but you should try to cut it symetrically and the drain needs to be as close to the center as possible. There is not much slop in where the drain can go unless you make a custom pan. Check out
www.schluter.com for the specs and a video on installing the stuff.
You should also check out
www.johnbridge.com. They have an on-line source for the Kerdi stuff if it isn't readily available where you live. If you want to avoid any caulk in the shower, schluter has some neat expansion joints designed specifically for that. The advantage of a Kerdi shower is it puts a waterproof membrane immediately under the tile so there is very little in a shower that can absorb moisture. The whole thing dries out much quicker which means less susceptability to it growing mold or leaking.