Plywood for Subfloor

[froddan]

I have an area in a room, approximately 10*10 ft, that is an add-on.
The joists in this area are about 3/4 inch higher than the other joists in the room, and level with the subfloor. This area was previously tiled over plywood about 1 inch higher than the rest of the room, which is hardwood.
I have removed the tile and cut out the subfloor USB board, and the joists are 24 inch o.c. , but the problem is that there are no room for a subfloor without getting the hardwood above the rest of the area.

I was thinking about sistering the joists, about 3/4 inch below the existing joist, and put 3/4 iinch plywood sheets in between the joists (24 inches wide) that would rest on the sisters so the plywood would become level with the joists.
Then I should be able to lay the same type of hardwood as the rest of the room and keep it all level.
My question is: Would 3/4 inch plywood work? And would the 24 inch be ok?
Would even USB work?