Adding a new field in TeamForge tracker is easy. Here is how you could do it.
*Open TeamForge -> go to 'Settings' tab.
*Click 'Tracker settings' from the left menu.
* Click a tracker name (ex: Defects)
* In the next page, click 'Add Field' at the bottom of the page.
* Give a name and select field type etc and save settings.
Now if you try to create a new artifact in that tracker, you could see the new field value added.