I would figure out what your IBUs per gravity is right now (using a calc or beerSmith) then figure out where you want to be, then you can use an equation like:
Intended bitterness = (current bitterness*volume + additional bitterness*additional volume)/new volume
Then use a calc to see how long/how much water you need to boil. Remember that water has a an alpha acid saturation limit, so boiling a ton of hops onto 2 cups of water won't help your IBUs, the 2 cups will max out their IBUs then not pick up anymore (so they won't add much to your 5 gallons).
Once you get that, cool it down and add it right to your primary.
That's just what I would think the best way to do what you're asking would be. But in all honesty, you might just want to let the beer ferment out all the way. Call it a dryer barley wine, and brew it again how you want it.