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.