I agree with Bob that you could insert the joists by shaving some width. Another approach would be to use joist hangers. Assuming you are trying to place the joists on a beam, just block along the edge of the beam with a 2x6 between the 24 inch spaced joists. Shave the block as needed to fit. Mount new joists with a joist hanger to the supported block. Use glue along top edge of joist to keep it in firm contact witht the floor (avoid knocks and squeaks). You can use the jack to increase upward pressure as you secure the second hanger under the joist.