You can enter it directly into the equipment profile as 'top off water in the post-boil section of the profile or, if this is not a typical or a one-off, you can add it the volume to the 'top off water' into the fermenter on the 'vols' tab of the recipe. If the volume being added is reasonably sized (>10% of total volume) then you will also need to adjust your BHE to reflect the lower extraction expected from the mash. Removing that amount of water from the mash will reduce the extraction somewhat. If you reduce it proportionally to the volume reduction, then you should be close enough as a starting point. Once you get a brew down, you can fine tune the BHE to reflect your actual values.