You'd generally wouldn't want to alter the theme to do this; it's better to do it in a plugin or via the overlay. We've altered spring-etc.xml to use the MergeableCollection class to add extra menu items to various places. The mergeIndex property can be used to set the position of the new menu item.
There's a great piece of documentation on this level of customisation here; https://community.jivesoftware.com/docs/DOC-52397
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I‘d say that if the menu item is always there for all users, then you could modify the soy/ftl file via the admin console.
You wouldn‘t be able to insert the link into the middle of a current list though, as Matt says you‘d need to add it into the spring file for that.
If you want the basic option, I think the template you need is