Can I use multiple wire nut splices on a single wire in a wall can?
The wires have to come 6" beyond the cable clamp.  If you can't get 2 hands into the box, the wires must also come >=3" beyond the surface of the wall or ceiling.   If they're too short you'll have to pull more length in from the wall, or replace the entire cable run.  

As long as that is true, you can extend a wire using competently executed wire nut splices.  Competent means it handles a pull-test with no problem, and does not require tape to hold together.