Is it possible to change a flat paint to a gloss paint by using some type of additive?
There is no such additive. At least that I have seen. Flat and semi-gloss paints are actually different from each other in chemical formula. They each need different amounts of pigments, extenders, binders, etc. If you change the amount of sheen in a paint, you would have to change the amount of the other components too.
No, I wouldn't add anything. Go ahead and paint whatever it is you're painting and apply a coat of some type of clear finish, such as varnish.
