Have you tried a sysex transfer? It's supposed to be possible from a 4-op DX9 to the 6-op DX7. In that case the DX9 operators 1-4 show up on operators 3-6 on the DX7. Some parameters that the 100 may have that the 7 may not would just be ignored by the 7.
Conversely, sending DX7 patches to a 4-op would result in whatever was in ops 3-6 making up the patch.Let us know what happens.mn