Handling Territory Assignments Outside of CRM

We currently have a little over 3 million accounts that we have assigned out to our sales reps. They do not want all of these account in CRM, and so we have a separate database that houses these accounts and the territory assignments. In CRM we currently have 220,000 accounts and when needed the accounts from our territory tool are brought into CRM through web services. When owner ship changes there is a process (through web services) that updates the corresponding CRM accounts ownership, and the Ownership of associated Opportunities, and the sale rep on