15 gallons from 10 gallons setup

I've got a brewing bud, and together, we split 10 gallon batches. Now, a third is joining us, wanting in on our awesomeness.

Currently, we have a 15 gallon kettle and 10 gallon coolers. How feasible is it to produce 15 gallons from our 10 gallon setup? Can we scale in Beersmith, brew it strong and top off to our desired volume? I'm sure hops utilization would change, but what about extraction?

We haven't been brewing all that long, and aren't looking to upgrade all the equipment just yet.

Just do two brews. One of you get 10gallons, the other two get 5. Done and done.