You've got 2 options really, the first being to create an invisible sprite that takes up the same screen space as the 3D object, and use that for checking 2D collisions. The second would be to create 3D invisible objects for each sprite, again taking the proper size into account, and checking for 3D collisions. Obviously the first is easier, but it depends on what you want to do, if you've got the time to create 3D models that are the same shape as every sprite, then adding 3D sliding collision to the sprites wouldn't be hard. 2D sliding collision is harder than you'd think, as in 3D, you have the angles of normals being hit, whereas in 2D all you have is pixel data.