Reducing sweetness of packaged cookie mix
Probably there is no such way, at least not one that's worthwhile. 

First, there is the problem that designing a recipe well is a skill which very few people have. Experienced bakers can progress to it, but inexperienced ones can make 100 trials but won't understand what went wrong with any of them. Unless you're in it for the fun of it, it's easier to find another recipe which works for you (you might have to bake through 4-5 failures to find a good one) than to redesign an existing one (might have to bake through 25-30 failures to make a good one). 

Second, you're not starting from a recipe, but from a mix. With standard ingredients, you know what went in, and can change the ratio easily. With a mix, you have no idea how it works and how changing something will tip the balance. A simple addition of one more ingredient (e.g. flour) will certainly not cut it. If you add enough to make a change in sweetness, you won't like the change in texture, which, for flour, will be rock-hard cookies.  

And this brings us to the third problem. Sugar is not a sweetener in cookies. It is a bulk ingredient which gives them a cookielike texture, and the sweetness is a side effect. If you combine the same ingredients but with less sugar, you'll end up with tiny cakes instead of cookies. 



Proposed course of action: forget these cookies. You don't like the taste, and you can't keep the texture while changing the taste. Baking cookies from scratch is not much harder than baking from mix. Go to a good recipe site, find a well-reviewed recipe for the type of cookies you prefer, and bake away!