This article explains how to update a non-inventory item to inventory and also include quantity on hand with quantity as of date.
1. Log into your QBO and click on the gear icon on top right corner
2. Select "Product and Services" menu under Lists section
3. Click on Edit link of the item that need to be converted to an inventory item
4. In the properties page, please change type link and change it to Inventory
6. Once the changes are made, please click on Save and Close
The above steps will convert the item into an inventory item.