Do you need more shelves, or do you just need one of the existing shelves to be at a different height?

If it's the latter, it may be easier for you to add some supports underneath one of the existing shelves like this:

![shelf with support][1]

The supports should run the depth of the cabinet, and you can attach them with [command strips](http://www.command.com/wps/portal/3M/en_US/NACommand/Command/Products/Product-Catalog/?PC_7_RJH9U523080QE0IIHEICN90GC1_nid=9ZKWFL6T55gs7NZP56G6BDglNJK0H1XK79bl) or double-sided tape.  Be careful removing them because shelves are often covered with paper that's printed to look like wood.  

  [1]: http://i.stack.imgur.com/MSA8P.png

If you need more shelves, it may be better for you to build a new shelf using the same idea, but have the new shelf resting on one of the existing ones.  You'd lose a little bit of space on the lower shelf, but you'd avoid the risk of breaking your rental agreement and/or losing your deposit.  You'd also need to be careful not to overload the lower shelf or the whole thing could come down.