Can you add seasoning halfway through cooking with sous vide

That is not a problem. If you are just lifting the bag, adding an ingredient, and returning it to the water it should not be an issue. You are not going to change the temperature in that amount of time. In this scenario I would suggest a ziploc freezer bag, rather than vacuum sealing, so maybe even double bagged. Normally for a long cook, I would recommend vacuum sealing. The reason that vacuum sealing in this situation is problematic is that after a few hours your brisket will release quite a bit of liquid. This would make re-sealing a challenge. I don't see a real issue here. Lift the bag, open, pour in your liquid smoke (not too much), reseal. You don't even have to take it out of the bath.

I would think that it wouldn't be ideal ... but if you work quickly it shouldn't cause too much problem. (basically, you want to work quickly enough that the meat doesn't cool down significantly).

I'd probably prep the replacement bag before I even took the meat out of the water, and have a towel and scissors ready (pull the bag out, dry the outside, cut it open, dump it into the new bag (with the liquid smoke already in it), seal it, then get it back in the water.

If you're using a vacuum sealer, and not just a zip-top bag, I'd make sure it's warmed up first, too.

Drying the bag might seem like a pendantic step, but it's both to prevent you getting water into the new bag, and to reduce the amount of evaporative cooling.

With the important exception of salt, there’s little to no difference between adding things to the bag pre-cook, mid-cook, or post-cook. Go ahead and do it, but there’s no reason to worry about it. All you are doing is adding steps and potentially cooling down your food (which is a risk).

It just doesn’t make a difference. The molecules are too big to penetrate the surface of meat. You’re just coating the surface of the meat which can be done at any time.

This is an unpopular assertion because it seems counter to the way most people cook and it’s not intuitive but he truth is, liquid smoke isn’t chemically doing anything to your meat while it is cooking. It’s just a condiment or sauce that you want to coat your food in. This would be different if you were cooking in some kind of chemically active marinade that is tenderizing the meat or something, but I don’t see many recipes that attempt this.

Another reason why this “feels” wrong is because we are used to cooking dishes like this as braises or other higher heat applications where the components of the cooking liquid itself can transform from the heat. For example, garlic or mirepoix in a SV bag will never cook at traditional temperatures. It’s just not hot enough to develop their flavor. But we are very accustomed to cooking them along with meat at higher temperatures.