Yes, you can fulfill your Excel orders without converting your file to CSV. When setting up your Data Feed, go to the Source Data section, and select Excel (.xls, .xlsx) from the File Format drop down.

Once you've selected Excel from the File Format dropdown, a new input box will display called Sheet Name.  Add the sheet name that EZ Fulfill needs to use to read your fulfillment data to this field.  From here, continue mapping the columns in your Excel file to the appropriate Shopify fields.  Once you've finished mapping your fields, you're all done.  Save or update your Data Feed settings, and you're ready to fulfill your orders.