Of your total grain bill, that third runnings will have a 1/4 or less of the potential gravity points. Designing Great Beers goes into detail on how to calculate this out.
Capping is easy, just throw those new grains in there before you sparge again. Keep in mind the additional water consumption of the new grains. Also, you'll want to stick to crystal malts or other high kilned malts and stay away from basemalts - as you will not be mashing these grains.
And the idea of having DME on hand in case your gravity is too low is spot on. Alternatively you could increase your boil time, but then you'll end up with less beer.
Yo, What's Wrong With The Beer We Got? I Mean the Beer we got drank pretty Good Don't It? I Ain't Never Heard nobody complain about the beer we have..... It Drank Pretty Good. Budweiser...What's the name of some of them other beers ?