I forgot to add the brown sugar to a cookie mix. I have added all the ingredients including the flour. Can I still add the sugar to the mix?
Cookie mixes (and similar mixes) usually call for the dry and wet ingredients to be mixed separately, which is mostly to prevent over-mixing and to properly control the amount of time that the baking powder/soda (or whatever chemical leavener is in there) is allowed to start its reaction before going into the oven.
Of course you can still mix the sugar in later, you're just more likely to end up with a cookie that's drier and/or harder, with a "grittier" texture, due to the extra mixing.
The bigger problem is waiting; cookie mixes are usually designed to leaven very quickly once mixed, so if you let the mixed dough sit for an hour before baking it, you might end up with very flat cookies. They should taste more or less the same, and certainly aren't going to harm you, they'll just have a weird look and texture.