I believe that if there is a neutral and a ground in the existing circuit you want to change that you would be creating a multi-wire branch circuit. You can't however grab a 'nearby' neutral to create this. If there is no ground wire with the existing circuit, you would need GFCI outlets at the start of the branch.

" share the neutral from a nearby circuit? "

NO.

If they had used 12-3 and had a Neutral there, you could do a MWBC. But not without it.

I agree. The answer is HELL NO. You can very much overheat the circuit you stole the neutral from. In addition, if there is an AFCI or GFCI in play, it would trip immediately anyhow.

Correct, you can't grab a neutral from another circuit. Not only is it a massive code violation, it's dangerous. You'd be asking that neutral to carry the current of TWO circuits, double what it's designed for. If you try that, better make sure your fire insurance is paid up.

If all you want to power is lighting, you can convert it to a 120v circuit. Remove any reidentification at the ends of the cable (usually it's black or red tape or paint on the white wire). In the panel, remove the white wire from the breaker and connect it to the neutral bar. In many panels, you'll have to extend the white wire for it to reach the neutral bar.

Replace the 2-pole breaker with a single pole one, and close the resulting hole in the panel cover with a single-width filler plate (available from a decent electrical supply jobber and sometimes from the big box home stores).

Don't forget that any splices you need to make to extend the existing circuit must be done in a junction box. Most likely your baseboard heater served as its own j-box, so you'll probably have to cut a hole in the baseboard where the cable exits, and install an old work junction box there. Be sure you have enough cable from the old circuit that at least 6" of wire extends from the box.

You'll then have to fish cable from that box to the new switch for your lighting. If you're feeding a shallow-buried UF underground run to the lighting, you can probably put the required GFI in that same box.. Otherwise, put a blank cover on it, or install another receptacle there.

It might not be cheaper, but would it be legal to use a 240V to 120V transformer? I assume Marion's 12/2 line has a ground.

Well, he says he's lighting his yard. If he's talking about low voltage landscape lights, he can just get a 240 to 12V transformer. If he's lighting it with other bulbs, it might be easier to get 240V lamps instead. A lot of the LEDs out there will run on anything from 120 up to 277 V