Yes, but you have to be careful. If the beer is already carbonated, adding pellet hops in a bag will cause it to foam excessively. Bag it up with some sanitzed marbles and drop it in. Then close it up fast.
I've done it. As Cbaddad mentioned, do it as fast as possible before it foams up.
One thing I will say is dry hopping in a carbonated keg (especially if it's cold) will take a while before you notice the difference. When I've done it I noticed as much as 1-2 weeks before I got any change in aroma/flavor.
Bier war sein letztes wort dann trugen ihn die Englein fort...
That does work, but you don't get anywhere near the same intensity of aroma or flavor as you do with adding the hops directly to the beer.
I've only done it once and 1 ounce of Simcoe "hop tea" into 4 gallons completely changed the beer. It added enough intensity and aroma to a kinda plain and boring IPA, so I could only imagine what adding it to the beer would have done.