Dear @Amr.Hegazy,
I’m not sure I fully understand your question, but the state-space wave-excitation model provides a state-space model between wave elevation and wave-excitation loads. If you already know the wave-excitation loads, there is no need for this model.
In OpenFAST, the hydrodynamic loads are output from HydroDyn and input to ElastoDyn. So, effectively, the hydrodynamic loads (summing across all hydrodynamic + hydrostatic contributions) are already included in the input vector of the full-system state-space model (when LinOutputs = 2 is enabled).
Best regards,