I've only brewed once with Malto, but I think you can still add it at anytime during the fermentation, without any issue. I would just boil it for 15 mins in a cup or water or so, let it cool, then add it. I believe it just adds some mouthfeel to the finished beer, as well as some residual sweetness.
From wiki:
Maltodextrin is sometimes used in beer brewing to increase the specific gravity of the final product.[4] This improves the mouthfeel of the beer, increases head retention and reduces the dryness of the drink. Maltodextrin has no flavor and is not fermented by the yeast, so does not increase the alcohol content of the brew.