I do BIAB and often push the volume limits on my 10-gal pot. If I want to add water after the mashed grain has been pulled from the pot before I start the boil phase does it need to be treated like the mash water was?
Not necessary when adding a reasonable amount of water post mash. You can add Sulfate, Chloride and Sodium to season the beer at any time, right up until packaging, as you would salt and pepper to your food.
Bob is correct. There are two reasons to add salts to mash water: 1) to get correct pH during mashing for efficient conversion, and 2) to get the proper balance for final flavor. Any water added after the mash only needs seasoning for final flavor. That may be a different treatment than the mash water got.
The mineral content of the makeup water doesn’t have to be adjusted, but it might be a good idea. The ‘have to’ items to adjust for your makeup water is chlorine removal and alkalinity adjustment. If you’re using distilled or RO water, you won’t need to worry about the ‘have to’ items since they’re already taken care of.