Hi chris. First of all you must make your new filltype, by adding it to the fillTypes.xml. Because the FS22 fillplane textures won't work, I reccomend to use the base-game silage textures or other silage textures from FS25 and name it GRASS_FERMENTED or GRASS_SILAGE or whatever.
Then duplicate a silage heightType and set it for this new filltype inside the densityHeights.xml, otherwise it won't work as a bulk.
Finally inside the scripts folder in the bunkersilo conversions just replace the original silage with these two new: base-game silage from chaff and grass silage from grass.
<bunkerSiloType attachTo="chaff" inputFillType="CHAFF" outputFillType="SILAGE">
<acceptedFillTypes beforeSwitchLimit="CHAFF" afterSwitchLimit="CHAFF SILAGE BEETROOT_CUT SUGARBEET_CUT PARSNIP_CUT POTATO_CUT CARROT_CUT MISCANTHUS_CUT" />
</bunkerSiloType>
<bunkerSiloType attachTo="chaff" inputFillType="CHAFF" outputFillType="GRASS_SILAGE">
<acceptedFillTypes beforeSwitchLimit="GRASS_WINDROW DRYGRASS_WINDROW" afterSwitchLimit="GRASS_WINDROW DRYGRASS_WINDROW SILAGE BEETROOT_CUT SUGARBEET_CUT PARSNIP_CUT POTATO_CUT CARROT_CUT MISCANTHUS_CUT" />
</bunkerSiloType>
Also in bales folder you must edit each bale to ferment the grass into grass silage instead of normal one.