I would not. I have put buffer in my sump where the water flows through the filter sock.
but is when I add just a little. now I have a 600 gal system so a little to me is a lot for a small tank.
no, do not do this. depending on the compound, you will likely have at least some precipitation which defeats the purpose of supplementing as well as would screw with other parameters.
also, IMO buffers should never really be used. you want to supplement alk via carbonate and bicarbonate. some of the buffers use borate IIRC and, besides being more expensive than baking soda, screw with your ionic balance.