You can manually add the email column to your Excel file before uploading it to QuickBooks Online (QBO), Felipe.  

Once the email column is in the file, QBO will automatically detect it during the mapping.

Here's how to import your invoices:

1. Select the Gear icon, then Import data.
2. Choose Invoices as the record type and hit Import.
3. Click on Upload a file to import data, then Upload.
4. If there are customers not yet in your QuickBooks list, check the Add new customers box.
5. Select Browse, find your invoice spreadsheet, then click Next.
6. Ensure you have mapped the correct column header for your emails and the rest of the information, then hit Next.
   
   
   
7. Click Complete import.

For more information, you may refer to this article: Import multiple invoices at once.

If you have additional questions about uploading invoices, you may reply in this thread. We'll be happy to assist you.