FM8 is MicroDexed^8!
MicroDexed is nearly full compatible with the “old” DX-7 (MicroDexed has no portamento, but 3 slightly different sound engines and less noise).
The DX7-II has a better engine and also less noise against the “old” one. You can also split or double voices (then only 8 notes per voice instead of 16), it has micro-tuning and some other smaller goodies.
Both DX-7 have 6 sinoid operators.
FM-8 has 8(!!!) sinoid operators, much more algorithms and features (AFAIK an Arpegiattor and effects). It is a FM-Monster and IMHO sounds really cool. But it is software…
Forgot to answer your question:
Yes - (IMHO) FM8 can import DX7-SYSEX data. But with MicroDexed you currently cannot edit sounds (only with external SYSEX editor like CTRLR - but this is currently untested).
The other way (FM8->MicroDexed) may be possible but only if FM8 can export DX7-SYSEX format and only without the additional gimmicks of FM8.