I don't see it being a problem as long as you boil it first, and pitch it as closely as possible to the current fermentation temperature (so you don't shock your yeast). However, you will be increasing your volume slightly with the additonal liquid.
I would not add DME to the fermenter without first boiling it.
Then you are fine. The yeast will eat sugar after fermentation has finished when they carb up bottles, so they'll also restart fermentation if you add a fresh dose of DME.