Tobacco in tins should age well providing the tin hasn't been opened. Round tins store better than square ones on account of the integrity of the round seal. Once a tin is opened for sampling, the tobacco should be good for at least several weeks or even a couple of months. Make sure to remove any bits of tobacco from the seal before re-sealing. Plastic covered 2oz. cans last about as long as round metal toped tins, after opening. But after a couple months you'll need to rehydrate the tobacco. And any blend will loose something after rehydration. I wouldn't add the contents of a tin to the same blend already jarred, as they'll be diffrent ages. Ball jars are cheap enough that you can dedicate them. They recycle well too. Just remember to use new sealing tops for each new jarred blend. Screw top rings good for several recyclings. If your paranoid about air leaks you can always dip your jars,top into molton wax to guarantee a hermetic seal.