can i use a stick of butter, to a already mix cookie dough? forgot to use 2 stick of butter

Yes! I’ve taken prepared dough out from the fridge (let sit 20 min) and melted 2 tbs of butter and slowly added to the dough. Use your hands to incorporate but avoid over mixing. The next batch came out MUCH better.

I would pitch the botched mixture and start over. As an ingredient, butter is relatively expensive, not something to waste.

As drbabs suggested, you should cook off a couple of cookies to see if they'll work out before trying to incorporate any more butter. If it doesn't work out and you remake the recipe, you'll be out four sticks of butter instead of just two.

You mean there's already flour in there? Probably not a good idea--in order to beat it well enough to blend everything together, you'll develop the gluten and end up with tough cookies. Why don't you try baking off a few cookies and see what happens? If you look at mrslarkin's Chubby Chewy Chocolate Chip Cookies, they came about when she accidentally put an extra cup of flour into the cookie dough mixture. They're a happy accident. Maybe yours will turn out ok, too. If not, no point in ruining a good stick of butter.