Hi there.
As far as I know, in Magento back-end you can sort unassigned attributes using drag&drop only, though you still need to move them to needed position.
If you are interested in alternative, you can try Store Manager for Magento application. Using it you can sort unassigned attributes by name and put them in alphabetical order with one click -
It is paid, but there is fully-functional 2-weeks free version you can download from the link in my signature and do the task without fees (in case this is one-time task) .