Is there any software that will allow it to happen?
I'm not aware of anything and I wouldn't trust anything that claimed to do it. I've seen "proper" hardware RAID controllers that let you do things like add disks to an existing array, but nothing to convert RAID 0 to something else.
My point of view is that you pretty much HAVE to back up in order to be sure your data is safe, and that that point why not just do things the "old fashioned" way anyway?
Do I have to create a complete image of the raid 0; reconfigure and re-format and then restore the image?
You don't have to do it as a disk image as such (though that might be the quickest method) but this is how I would do it.
I currently have 2, 640 gig drives running Raid 0. About 300 gig is currently stored on the array. Would the back-up image be 300 gig or 1.2 terrabytes.
Oh... 300Gb. You back up the files you have, not a bit by bit image of the disk (which would only be 600/640Gb anyway as this is the size of the logical drive that is sitting on your RAID 0 stripe).