3 wire conductor to an electrical subpanel - Can I add a separate conductor for neutral in same conduit?

I’m installing a subpanel to bring power to a garage. Answers to similar questions make it clear that a subpanel must isolate ground from neutral, so the bonding screw will be removed at the subpanel. The standard recommendation is to run /3wg cable from the main panel to the subpanel.

But I’ve got some NM-B 8/2wg cable that I’d like to use. I believe that would allow:

• Either 120v service (a single pole breaker on the black wire at the main panel, white as neutral)
• Or else 240v service (a double pole breaker on the black and the red wires – having taped white as red, with no neutral)

But I can’t have grounded circuits of both voltages at once without a transformer.

Can I use my 8/2wg and simply run a single #8 insulated copper as a separate neutral conductor beside it to get both 120v and 240v service at the subpanel?

If so, must both the cable and the single wire be within a conduit? (the run is in a dry interior, through framing covered by sheetrock). The total distance isn’t particularly long – about 30′.