Are you having problems with flats too frequently for comfort? If not, leave them alone.
If you are, whats causing the flats? Punctures? Too low tire pressure? If its the latter, sealant won't help.
But, people do put Stans sealant into tubes to do the same purpose like Slime to deal with punctures. It comes with the usual caveats just like Slime (possible gumming up of the valve which can cause air leaks at the valve, the movement of the sealant, etc.), but for people who get punctures which are sealable by Slime frequently enough, it can be useful. Stans has a different viscosity and drying time to Slime, so its probably not as good (and its probably more costly to use Stans for this purpose than Slime).
So, yes it would work, maybe not as well as Slime-like sealents, but it would probably be more economical just to buy Slime and use the Stans for your tubeless applications. I think if you're not getting punctures frequently enough to be annoyed, its probably more trouble than its worth to use a Slime-like sealant in the first place.