In modeling beer recipes, BeerSmith looks at all fermentables as sugar by weight that is then diluted into your water. When adding juice, BeerSmith doesn't account for additional volume, only the added sugar content. To account for volume, you have to put top up "water" into the equipment profile, choosing pre or post boil times. This will make the sparge volume change by the corresponding amount.
If the addition is substantial, you will probably have to change your brewhouse efficiency to account for lower efficiency from the grain.