The custom code is only required to let GSX know if a door is open or close. If you prefer a belt loader, just change the loader type in the airplane configuration page.
However, keep in mind that GSX supports 2 cargo loaders on the right, and 1 main cargo loader on the left (for cargo planes). So, you cannot "add" a belt loader, you can only replace one of the two ULD loaders with a Belt loader.