It's my understanding, perhaps incorrect, that you are NOT supposed to run Romex inside of EMT. It's also my understanding that you are not allowed to strip the individual wires out of Romex and use those inside the EMT. You are supposed to use THHN, I think, wire. The wires inside the Romex sheath are not THHN wires, right.
The case is where you are running Romex up in the rafters, then coming down a concrete wall to a metal box. If I remember correctly you need a box in the rafters to transition from Romex to the THHN wire and the EMT. Then the EMT running across the hard surface wall to the surface mounted box.
Am I correct? That's what I remember being told in the past. But of course I might be misremembering, or been told worg!