Adding fat sacs to existing Nautique ballast syste

Has anyone had any experience adding additional fat sacs to the factory ballast in a Nautique SV 211? I am trying to add two fat sacs to the port side that will sit under the walk through compartment right over the factory ballast and under the seat just in front of the walk through. I have input, output and vent for both the factory ballast and the new sacs. I have been told to fill through the factory vent but I think there must be a better way. Any advice is appreciated.