Hi, It's possible. The adaptor is available. Just search for a 2 1/2" to 3 1/4" hard drive adaptor. They can be had with the brackets to mount the 2 1/2" into a 3 1/4" drive bay, but the brackets may not work in the external enclosure. Price is usually in the $10.00 range. 2 1/2" enclosures are sometimes around $20.00 at the computer shows and wont need the adaptor. Check around.
Good Luck, Jim