Dear Purushotham,
What sort of FAST - ANSYS/AQWA coupling are you envisioning?
If you simply want to take the frequency-dependent hydrodynamic coefficients computed by AQWA and use them in FAST/HydroDyn, you will simply need to convert the format to that of WAMIT, and perhaps shift the reference point as discussed in the following forum topic: hydrodynamics - #4 by Jason.Jonkman.
If you want to develop some sort of two-way structure-to-structure coupling between FAST and ANSYS/AQWA, that is a much more involved task.
Best regards,