In many municipalities do not allow romex in conduit, so check local code. I know the west coast states do not allow romex in conduit even if you up size the conduit. Reason being, it increases heat loading on the wire But ,more to the point kind defeats the purpose of the conduit, if you run conduit then run bare wire, it is better assembly, the wire will dissipate heat better from the wire. But, if you are tying into romex, then, just terminate the romex in a 4 square box with a romex connector and the then go from there in conduit. That is most common. If the change to conduit and it is less than twenty feet, you could just strip the white/yellow outer plastic carefully cut out the paper insulation wrap and tension pull paper cords away and feed the now separate 3 wires into the conduit alleviating the connection wire nuts in the 4sq box. If you go this route, remember to leave about 8-10 inch service loop in the box, in case you have to replace the conduit side wire or to need extra slack from the loop. I do this a bit but usually, on runs that are fairly short less the 10 feet. My conditions are different, we are under contract so for us time is money.
The other thing, code requires in basements, garages and out buildings the wire be protected if it is in the wall lower than .......I believe it is 80" like the door height. This can be accomplished by sheetrocking or plywood the wall so the romex is not visible (or just that stud bay). Often, in these conditions, we will go to MC cable (Metal clad) it has 3-12 ga wires- black, green and white. It is a roughly double the cost of romex typically, 140-180.00 per 250 ft roll. It is a useful alternative, but takes getting used to and you really need a roto-zip hand tool (mini cable cutter) to do it efficiently. Just mentioning so you know what it is and knows it exists. We use this method usually by running in the make-up boxes (4-sq)that feed power from the panel and pre-make the down the wall whips in MC with the outlet boxes attached all done in the shop.
Out here in Hawaii, I am currently working with an electrical inspector from North Carolina, I ll ask for you his thoughts. His electrical code knowledge is excellent..... not all inspectors usually this good. But, like me and so many of us old-scool boys we came up with tools in our hands. Now, so many inspectors and new managers are coming from college without that experience .......... cest' la vie