I use quick-crete all the time to refill holes in concrete. Although I'm wondering why you would refill it. You need to have access to a tub drain for repair. If you burry it in concrete how will you clean it or change it out when it goes bad. Most plumbers leave the hole open under the tub just for that purpose. In addition the tub waste and overflow sits down just slightly below the surface of the concrete.
I would suggest leaving access.
Ray Vin Zant