The grain bill would be adjusted according to your mash efficiency, which is controlled by the brew house efficiency you entered into your equipment profile. You would need to know the efficiency of the original recipe to figure this out or what I do when entering a recipe from a source which does not state the efficiency of the recipe is to enter the grain bill and hop amounts per the recipe with my equipment profile and then use the sliding scales below the ingredient box on the design tab to adjust the recipe to match the original for gravity, color and bitterness.
If this is your first time using BeerSmith for your recipes and you normally scale up 20%, I would scale the grains according to your standard procedure when you enter a new recipe and then once brewed, BeerSmith will calculate your actual mash and brew house efficiency on the 'session' tab once you enter your measured gravities and volumes. You can use those numbers to adjust your equipment profile for future brews.
Personally, I have never had to adjust a grain bill up for full volume BIAB. I grind my grains on the fine side and get about 86% to 87% mash efficiency. I actually need to scale the grain bill up when moving a recipe from my BIAB system to my mash tun set-up where I only get around 82% mash efficiency.
