I'm not sure I'm qualified to say this, but the only way I know of is to use a gloss paint of the same kind and box all of your cans together. If it's not boxed well, you will have patchy areas, which outside might not be seen anyway. Remember, adding 1 gallon to another gallon is a very small ratio and when adding it to 5 gallons, it will practically be next to nothing. You might be better off just getting the sheen you want in the color you want and use the old paint on something else. One thing you don't wanna do is to hafta do this over in a year or two, cause you cut some corners. Adding some Flotrol will give a little extra sheen, but not much.
Good luck on that one
edit : for Old Paint's comment, you're right it won't fade any faster or slower, but flat paint will chalk out far faster vs. when using paints with varying degrees of gloss/satin..... far faster.