I've added flavor to loose in the past and it worked pretty well. Just make sure that the starting snus is very generic in flavor otherwise it will be quite muddled.
I always used wizardlabs.us for vaping flavors.They have different brands for really decent prices. The flavors might taste bad for you in snus because they are extremely concentrated. Try mixing them down with PG. Even tasting a flat vaping juice can deliver really intense amounts of flavor hen you consume the juice directly.
Wizard labs has some good stuff. I also used to use the perfumers apprentice. I just don't know how much the pg will mix with the snus. I know people used to make e-liquid by using pg to soak portions and that residual juice is what they would vape for flavor and nicotine.
Read the back label of your snus cans. PG is E1520 which is literally in every snus as a humectant. The American General cans even state PG specifically.So it should be the ideal carrier liquid to flavor snus.On the other hand distilled water or everclear should work just as well.