Can I connect a 120V outlet to a 240v well house supply?
You are thinking of a tap rule and that would not be safe in this case because you only have 3 wire no neutral. There are 220v heaters but I would suggest good insulation on the pipes and letting a faucet trickle so the pump kicks on every 5 minutes or so, I have done this in several homes and not had frozen pipes even in the low single digits and -f  but I did loose some plumbing in the barn that now has heat tape luckily we had water in the house and could shut the feed to the barn off until we got the pipes fixed.