Plug it in and upload your music.
You don't need to have two separate accounts for iTunes on one computer, me and my 3 roomates all share the same computer and iTunes library. Separating them only causes headaches.
If you really want to though, if your on a windows computer you can create another user for the computer by going to my computer and add a user, then once in the new user download iTunes and upload music. One thing to remember with this however the iPod and iTunes does not support fast user switching (meaning when switching users in windows instead of logging out, just going to switch profile.)
Hope this helps,
Ms D.