Is there a reason you would want to use a different account? You can have (I think) 6 devices authorized to your Adobe ID. In addition to my computer I currently have 2 Kobo readers authorized, and previously also had 2 Sony readers. All my books can be copied to either reader, or a new device should I drop one into a lake (again).
how do you do this
First make sure you've already authorized your computer to your Adobe ID (under Help in the upper left corner of Adobe Digital Editions). Then plug in your reader with the USB cord and tell it to Connect to the computer. After a minute, you should see the reader on the left in ADE and there's a dropdown list of things you can do, including authorizing or deauthorizing it to your Adobe ID. caveat: I have version 2.0 and I assume that version 3.0 is basically the same.
If you don't have your computer or readers authorized, then only the one where a book was bought or borrowed (i.e. a library book) 'owns' it and it can't be copied anywhere else. If you buy or borrow something on an authorized device, then your Adobe ID 'owns' it and it can be copied to another device authorized to your ID. I have a library of 1000+ ebooks and have my library backed up in case my computer dies. I can get a new computer, reinstall ADE, authorize my computer to my ID and import all my books again and everything will still be readable (it's happened...)