How to add custom field to NS inventory

Does anybody know how to add a custom field to an NS inventory that would keep track of the purchase date for our systems?

I can create a file on each system, or a reg key, or something that would be scanned by the inventory process but I need a way to get in captured in the NS inventory and populated to a field in the DB for reporting. Currently we manually enter all asset info into an old access db and I would really like to make that ugly thing go away in favor of real time info pulled form Altiris reports.

Help!