I think it would be fine to add about 1.25 pounds of DME but it will also affect your hop balance. You should really add a proportional amount of hops as well to keep it in balance. What is the AA% on the hops they sent you? If it is an ounce of pellets at anything between 5% and 6% (leaf at 5.5% to 6.5%), you are fine with what you have. If they are 4.5% or less (less than 5% if leaf hops), you will need more.
Kegged: Barrel Aged Imperial Sweet Stout, Rye Ale, Old Ale
Secondary: Barrel Aged Imperial Sweet Stout On Cherries
Primary: Fruitcake Old Ale, RIS
Next Up: White IPA
Projects: Brutus Strut-stand (Done), Freezer Conversion (Done), HERMS (Done), Lager Fermentation Mini-fridge Extension (Done)
Drinking: Great Lakes Edmund Fitzgerald, Fuller's 1845, Founder's Dirty Bastard, New Glarus Moon Man, Fat Head's Hop Juju