Hmm, if you leave it I guess you'll just get a slightly lower percent of alcohol but it wouldn't be too bad. I just used beer engine with a recipe and then added 2 liters of water; the bitterness went down only a little bit and the alcohol percent dropped to about 3.5% but with yours it'll depend on what you put in it I think.
Adding sugar will help though, may need to add a bit more yeast too, just to make sure it all gets eaten up
