Conduit size, wire type

I have used 6-3 NM-B for a generator.



I want to protect it outside. I am finding that it may not be the correct wire type to put in conduit.



I can remove the black exterior sheathing and just keep the wires, but with no info on the wire insulation, I am thinking running actual THHN is required.



What is required and does that differ from what can be done with what I have?



Thanks