Q: The web team at our company wants me to figure out the best way to add a 6th level of navigation to our website. Is the best way to do this to add this level of navigation sub-nested under the 5th level on the left-hand side of the page?
No. The best way to do this is not to! There are very, very few instances where 6 levels of navigation are needed. That kind of layering makes it inherently difficult for users to be able to find the information they need. It sounds like it might be time to re-examine the entire navigation scheme and information architecture, and see where you might be able to pare down or regroup.