Can you make flat paint glossy?

just buy gloss paint

What is the item? Depending on WHAT you want glossy, there are a few answers. There are spray clear, glossy paints. There are paint on clear, glossy paints.... But the type of paint under it matters too. So what item, and what kind of flat paint is it? Like, is it oil; is it acrylic?

Cover it with a water based clear coat, if you have some. The water based will not yellow where as an oil based one would.

there don't appear to be any additives to change flat to gloss but you can add some gloss to the flat to make more of a sheen.

You can seal it with a polyurethane or the like to give it a nice sheen