I made two extra batches of buttercream frosting about a week ago when I realized it might be too soft. I have been keeping it in the refrigerator and plan on using it to make Halloween cupcakes this coming Monday. I would like to take it out of the refrigerator sometime on Sunday morning to get it to a room temperature and add more confectioner sugar to firm up the texture to decorate the Halloween cupcakes. Has anyone done something like this? What would you recommend me doing to firm up the texture short of throwing it away and starting over with new material? Any advice would be very appreciate it :) 

Sure :)  Just add your additional powdered sugar a little at a time.  Not knowing how much icing you have to start with I can't say how 'little'.  For a batch that used 2 pounds of sugar to start with I probably would only add about 1/4 to 1/2 cup more sugar.  NO need to throw it away.,if it's ABC just add more sugar to firm it up.

Thank you for the advice, kakeladi! Very much appreciated!