By default, metgrid.exe reads in only what is called intermediate format. WPS only comes with a conversion tool from GRIB to intermediate format.
In the climate community, we use the netCDF file format more than the GRIB file format. So some of you might have to write their own interpreter from netCDF to intermediate format.

You can find a good example of an interpreter in this git repository:
Note that this case was to create an intermediate file with only 1 variable in it and the configuration has to be done within the source file for now.