maybe i'll say a wrong answer but:
impossible with hise.
FM piano are based on 4 or 6 op FM (like DX-7 for the 6 op)
but the 6 op (oscillators) are crossed each other by following a matrix modulation that you can't reproduce with HISE.
in the waveform group you have the FM function, but it will just take 2 osc (waveforms)
you could create something pretty close with only 2 osc, but not the famous DX-7 patch 11 FM piano.
for the 4 op it's like that:
and the DX-7 6 op: