Looking good. Yeah you can add and rebake sculpey. You do need to be careful when sanding though, baked sculpey is pretty strong, but as this is a hollow piece (not sure of the wall thickness), it could collapse if you go at it too hard, especially if there are any flaws where you have added clay, and I'd hate to see that happen. So just go easy when sanding, not too much pressure, if you know what I mean?
Al