| I doubt it. The gloss of a paint is determined by the ratio of vehicle to pigment and the types of pigment. You won't find more of the same kind of vehicle as in the original at the local paint store. Even if you did, the higher quality paints use pigments with needle shaped particles that are very hard to make more glossy. Mixing in a high gloss latex enamel might work, but the vehicles could be different and not mix well.
You may find a mildew additive at the paint store. | 
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