Can I extend a cable to add another outlet?
It's easy and legal to retrofit a ground wire.

Since you have the walls off, get yourself some ground wire and run it back to the panel this circuit is served out of.   done.  

Get  more knowledge, though

Take a few evenings to learn this craft properly.  Get a book that starts you at the beginning and works you through it.  That's one virtue of big-box home improvement stores, they have books.  So do libraries.  Find a book that speaks to you and read it recreationally. Learn more than you think you need to know.