I have an existing plug which already has a switch connected to it. Can I connect another set of wires to the outlet to feed power to another new outlet?


You may, but unless the old outlet is a half hot - half the outlet remains hot no matter where the switch is - it will be switched as well. Or, if the power enters the outlet box, then goes to the switch and back to the outlet, the new outlet could be made permanently hot by using the incoming wires rather than what is terminated on the old outlet.

Updated on January 8, 2018

Original Article:

Adding Electrical Outlets:  How to Wire a New Outlet to an Existing One
By Dan Harmon