Forgot to add Sugar in cookie dough . Is there any way to add sugar?
There are tons of things you can add to them after the fact:


Icing
Jam
Crème au beurre with lots of sugar
Whipped cream with 2 packs of vanilla sugar instead of one
Grated chocolate
Honey
...


With some of the above (like jam, chocolate and honey), you can put them into the oven again at a low temperature (50°C / 120°F) and get a unique cookie!