There is nothing that can be added to increase the gloss of the paint. You will need to purchase a new can of gloss paint.
One of the things that make paint glossy or flat is the drying times. The longer it takes to skin over the more gloss it has. Also the chemical make up of the paint helps determine this as well. So there is little you can do to change this.
What are painting? If it's furniture you can add gloss after the fact by using a wax or polyurethane finish.
Everyone here has your two options covered. . . either buy it as gloss to begin with or add gloss via a top coat/ sealer.
There are lots of options to add gloss if you are painting furniture. You can even wax the walls over flat paint to add a little sheen- but buying a gloss paint would be a lot easier on walls.
I'm painting trim, Windows and doors. I was affraid of having to buy more white paint :/
Thank you all for replying.
You will have to get some more paint for that. Be carefull and make sure if what your painting over is latex or oil.
You can paint Glaze over it. Look up Benjamin Moore Latex Glaze #405 (it's semi-gloss), may go on milky white, but dries clear. (Indoor use only). Glaze is generally mixed with paint to create Faux finishes such as sponging etc., but you can use it as a top coat. When I worked at a paint store, we would tell people to buy the gallon, open carefully not damaging the can/lid, take a small sponge brush and test a small spot/brush stroke, then use rubber mallet to put the lid back on. If you don't like it, you can return it. Investment if you don't like it, 1 sponge brush (less than $1)
Thank you Clay B I'll look in to it!