I have no direct experience with FV3, but assuming that it's output is natively on a cubed-sphere mesh, then at a minimum I think you'd need to interpolate the FV3 output onto either a latitude-longitude grid or a Gaussian grid before writing the fields into the same "intermediate" file format that is produced by the WPS ungrib program.
I may be mistaken, but I think the GFS model output that NCEP provides from the FV3-based GFS (FV3GFS?) has already been interpolated to a latitude-longitude grid before being written to GRIB format.
