Yes, you certainly can.

When creating a Data Feed in EZ Inventory, there's an option under Feed Type for Manual File Upload:

The only required columns are the product/variant identifier (such as the SKU or Barcode) and the quantity column.  You can include other columns in the CSV/Excel file, but these are the only ones required.

Once you've configured the Data Feed, you can keep re-using it, so you don't need to do this step every time you need to upload an updated version of the same file. Just upload the new file and click the "Start Inventory Sync" button.

Click here for a more detailed guide.