Not directly. Designing a new tree can be very complicated in some cases, even for a person who knows the system well. In order to make the task as easy as possible to the users, it works like this:
All a user has to do is think of an item, and then of a single ranking for this item. He can then repeat this step if he wants to add the item to some other ranking(s) as well. Now, the difficult job of how to arrange the tree is to be done by an OpiWiki moderator.
For example, a user wants to add Muhammad Ali to boxers and heavyweight boxers rankings (assuming these ranks don't exist). He just creates one ranking at a time, not worrying about the scheme.
If a user is experienced enough to have an idea of how to arrange the tree, he can propose this in the info for a moderator field (create new ranking menu).
If you believe some scheme could be designed better, you can propose a change, here, on Meta.