Using Area in TFS work item templates

Migrating to Conchango scrum template, I've lost the fields for epic and themes on my product backlog. You can guess how much I long for Rosario's hierarchal structure for work items. This would truly solve my problems with epics, themes and small stories.

The problem is adding fields to the template makes upgrades a hardship. So we do not want that. Concatinating the epic/theme into for example the description field is not a good option: I use epics and themes for making pivot tables in Excel and that would just not make it worth it. The reason for me using pivot tables in Excel is that I can make all kinds of analysis of for example cost per epic or theme (to calculate if the functionality was worth the price).

It is also a good thing to calculate the ration of bugs on different themes or epics to know which areas are prone to be infested. Which should make us aware of which areas should be better automatically tested when changing the stuff again.

But back to the problem with epics and themes. As from Monday, I store this in the Area field on both bugs and product backlog items. On the highest level I keep the themes and under each theme I've defined a number of epics. Since the field is hierarchal, I haven't written the epics as true epics as described by Mike Cohn but rather a few words (for example theme resource planning with the epics choose the right resource and handle sick leave.
This is not optmal, since an epic can belong to many themes, but the cost for another solution is frankly too big.