if you're looking for an additive to increase the life of the fuel. the 2-stroke oil does that - it adds the carbon backbone molecules needed for the fuel to not break down as quickly. however fuel will still go bad eventually
if your looking for an additive to lubricate the engine, the 2-stroke oil does that.
if you are looking for an additive that does some miracle thing, it doesn't exist.
if you are looking for an additive that cleans your fuel system, it doesn't exist
there is no mechanic in a can
therefore the only and I repeat the only additive you need for a pre-mix 2-stroke fuel is the 2-stroke oil.