No, I just did that myself. I had two kegs of a (supposed to be) hoppy beer, and it was good but it was lacking. I just dryhopped the other keg. You can use a stainless "tea ball", the large size that holds about .5 ounce of leaf hops, or the hops bags that are tightly woven to hold either leaf hops or pellet hops.
Dryhopping "works" faster at room temperature, but it will work either way. If you're doing it at room temperature, hopping a week before serving would work out great. If you're dryhopping at fridge/kegerator temps, then do it sooner rather than later.
Just purge the gas, remove the lid, gently drop the hops in, and cover and add the gas. Purge once or twice, to get rid of the 02, but then don't purge after that if you don't really need to because you don't want the hop aromatics to blow off!