I understand that you want to use the excel file as the input for the columns to be imported.
If this is the case you can use the button ‘New Template by excel file’ to upload your excel file and the list of columns will be automatically created.
See the documentation here: https://docs.mendix.com/howto/integration/importing-excel-documents/#7-automatically-creating-an-import-template-via-an-excel-file