@lightface79 Are you sure that's really a 120V DC power supply? What are you running with it?
As for "will the 120V power supply switch to 240V", only you can determine that. SoME can, some cannot, and still others have a switch.
You might get more replies if you list out your questions separately, but I would start with the data sheet of your power supply. Messing with 240VAC is fairly serious stuff, and can make expensive electronics disappear in a flash. We won't be able to answer any questions without more information, and even there, anything power related is sketchy. To be blunt, just the fact that you're asking is an indication to me (electrical engineer) that you best have someone who knows more look over your plans.