Can I run stripped Romex/NM through conduit?
The wires in NM aren't labeled for use outside the cable jacket, and may not be the correct type for use in conduit to begin with
First off, the wires inside a NM cable are not marked or labeled at all, which automatically makes them unsuitable for use in a conduit wiring method, as NEC 310.120 requires conductor insulation to be marked/labeled with the correct insulation type letters, the rated voltage, the wire gauge, and the manufacturer.
Furthermore, the wires in NM are not required to meet any of the type standards in Table 310.104!  In fact, while UL 719 section 4.4.1 permits the use of THHN type insulation on wires in NM cables, it also permits the insulation on said wires to meet the standards for type TW wire.  It even allows cablemakers to use an insulation material that's not PVC at all, provided it's electrically and mechanically comparable to type THHN insulation and meets the aging requirements set out in UL 1581 Table 50.144.
As a result, you have to treat the individual wires in NM as if they were &quot;mystery meat&quot;; once they have departed the cable jacket, they can only live within a single box, either as wires emerging from a cable that enters the box or &quot;shucked&quot; from their jacket for use as pigtails or jumpers that never leave the confines of said box.