DME and LME need to be boiled and stirred so that it completely disolves. I understand that you don't want to add any volume to the fermenter, but the best suggestion that I can make is to mix a small amount of water with a little bit of DME and mix to disolve, then add a little more DME and mix to disolve. Do that until you have a small amount of very high gravity wort. Doing this will require at least a half gallon of water.
With that said, if I were in your shoes, I'd just leave it. A 1.055 beer is still going to be a pretty decent beer.
Bottled: Oatmeal Porter, Double Chocolate Chipotle Porter
Kegged: IPA, Red Rye