Ok fair. Two 120V rails in series would supply 240V but I think the dangerous part comes from the 'also' and that's where an electrician would come in and turn this from 'just a breaker' to rewiring your house.
you will also need to replace your receptacle
True.
You will also need to upgrade to 12/2
True.
I just don't think it's safe to say 'yes just add a 240V breaker and plug it in' which appears the OP may be at their with level of understanding.