One could buy; hopped extract kit. and bottled water.
Here are some hopped extracts!
Then boil water (1 gallon), shut off heat reconstitute, stirring in extract, then top off to 2.5~3 gallons of ice cold bottled water and ferment with yeast that comes with the extract kit .
It would be a higher gravity fermented concentrate that could be watered down with steeped grains and then aroma hops.
Use a Better Bottle with racking attachment.
http://www.better-bottle.com/
Weeks later fill your Whym Jug with the base beer and add the steeped grains with aroma hopped water. Carbonate.
You are basically doing a blend that old European breweries do. Namely small Belgian Breweries making lambics, oud bruins and a few other styles. But i think they are blending a high OG and low OG fermented beers.