Customize Admin Sidebar Menu Items?
Craft does not offer this functionality natively. Luckily there's a few plugins out there that will help your users navigate around a bit easier:

In the CP Nav
https://github.com/engram-design/CPNav (This should accomplish your need)
https://github.com/picdorsey/craft-sidebarenhancer

In the Entries index
https://github.com/james1238/EntryTypeSources
https://github.com/engram-design/ExpandedSingles