When moving an existing outlet for a light over a sink to an overhead light can you pig tail copper wire onto the existing wire? Or is there something special you need to do first?

Read this Aluminum wiring (http://www.faqs.org/faqs/electrical-wiring/part2/section-16.html) I don't recommend doing this but sometimes you have to connect these 2 wires. Solder or a crimped collar can be used but not just twisting and tapping.

As reference points, please see NEC 110.14 "Electrical Connections" and 110.14 (B) "Splices".

At the top of this forum is a link to the code site.