derSammler wrote:

That's not really possible, simply because FM synthesis doesn't work that way. You can't form a waveform using FM synthesis to resemble an arbitrary sample. Not with the limited operations of the OPL2/3 anyway.

What I meant by "provided that FM synthesis can produce the sound in question" is that two-operator FM synthesis obviously cannot produce sounds with each and every spectral envelope.
However, if the input MOD sample has a spectral envelope that two-operator FM (or the summation of let's say two such channels) can reproduce, the output would sound vastly superior to crude 6-bit PCM.
A separate step would find appropriate ADSR envelopes in the time domain.
As a result, a pipe organ sample would sound roughly like pipe organ, a guitar sample roughly like guitar and a drum sample roughly like drums.
The exact waveform should be largely irrelevant. Lossy audio compression does not usually replicate that, either.
This technique would basically just automate what is otherwise a manual process done by the composer of an FM tune.