With Abigail, I use a paci just to settle her down before naptime.
I bring her to her room, sit with her and put the paci in her mouth, and put her down when she is calm but still awake. I put her down with the paci but she usually spits it out and finds her thumb.
As far as extending naps, I have NEVER had any success with using the paci to extend naps.
Also, Abigail never wants her paci any other time, just to calm down before naptime and bedtime.
HTH