How to connect metal conduit to in-wall NM cable?
It sounds like you're sheetrocking over the existing wiring.  If that's correct, I think this would work.  The challenge is you want to make the transition from conduit to plain NM in a box, and you want the plain NM concealed inside a wall, not out in the open where it's exposed to damage.    


remove the box from the end of the conduit 
install two boxes in the face of the new wall, maybe one at switch or sconce height, one at receptacle height.  (These boxes will be accessible through the new sheetrock.)  
Cut the existing NM mid way between the new boxes
extend the conduit to the upper box (with the NM in it) 
splice in a new piece of NM from the upper box to the lower box