@scott92
Is there a way to import inventory into preexisting categories?
No, you can't!
Currently, map data is not available for the Category field in QBO.

But after the import, you can Assign Category in a batch action from the Products and Services page. It's simple to update.
Just select the items you want to assign category from the left box, and the "Assign Category" tab should pop up. See the screenshot below.
