What was your intended (recipe's) OG and what is the current OG?
Did you add any (top-up) water to the wort in your fermenter?
Did you lose much wort somewhere in the process, like left behind in the mash or kettle?
As
@Steveruch said, you can add DME to it, anytime.
Just dissolve it in some hot water, in a kitchen pot, then bring to a boil (or at least to 170F for 10') to pasteurize, cover with a lid and let chill in the sink or a tub with cold water., before adding.
Use the
minimum amount of water for your makeup wort in order to keep the gravity of it as high as possible, while adding the least amount of volume.
That will fix this batch, now you need to find out why you missed the intended OG...
Coarse milled grain perhaps, wort losses, incomplete conversion, etc.?
It may take a few brews to get all your volumes and numbers worked out.
Unless you missed many points, don't sweat it too much, and use an extra pound of base malt next time.