With extract - you don't need to worry about pH as you are not mashing. You may know the mineral content of the water you are using, but you really don't know about the mineral content of the water that was used to make the extract. So, there is a certain amount of minerals already present in the extract itself. It might be high, it might be low - but you don't know what it is. So, adding on top of it is a bit of a guessing game.
That said - you can absolutely add minerals to the boil kettle for the purpose of flavor/perception. You can even add to finished beer to get a feel for additions. If my mineral additions are going to affect my pH in a way that is too extreme, I will add some of them to the boil kettle. If I was adding to extract, I would start maybe on the lower side a bit...... say 100 chloride and 50 sulfate for instance and see what you think. You can raise or lower on future batches based on your preferences.