Add a custom field that is viewable from both the project level and the milestone/task level

There are a couple ways to do this, but in either case I would recommend hopping on an advanced Q&A forum to have someone guide you through it (http://webinars.clarizen.com). You could either use a work item custom field with a workflow rule (and a scheduled workflow rule to backfill all of the old data) or a reverse reference field.