You can, with a bit of splicing things together. Indeed, filling the tank this way would be a good benefit that would make the extra cost and installation complexity of the cassette toilet, versus a Porti-Potti, well worth it.
By the way, I didn't take the time to look up whether your model is electric flush, but if it is, you might also consider connecting the pressured water up directly to the flush, so that when you press the button the water flushes with line water rather than tank water. In my first Sprinter, space was so tight that we didn't really have room for the flush water tank. By connecting directly to the flush plumbing, we were able to have a cassette toilet without needing the space taken up by the tank. (Of course, it required fabbing a custom fiberglass piece to cover up the rude look left after cutting the expensive toilet apart.)