I added to a pale ale that I made. It was a similar situation to yours and I just used about 1/2 gallon of water and a pound of dme. It really killed the hop character and gave the beer a homebrew taste. It could have just been that I knew it is in there so I thought I could taste it, but I won't do it again. You will still get a 4.x% beer out of yours which is still decent, and it should taste good!