It's not easy true. If you can get the source to compile locally then I'm sure it would be possible for someone to produce a patch to modify the firmware.
But you would have to lose a standard waveform to replace it with the new one, your firmware would then be incompatible with presets (not always such a bad thing
).
To be honest, FM is less about the starting point (the waveform) and more about modifying the waveform with modulators and algorithms.
It seems Operator is concerned with filtering waveforms and applying some FM, which is close to the subtractive way of making sounds.