How to add a new field to existing XML Mapping

Thank you very much, Charles_Kenyon​!

All great contents, they absolutely got me there! What actually got me to the answer was https://learn.microsoft.com/en-us/answers/questions/5029965/xml-location-of-document-properties-in-quick-partsthat%20list%20-%20Sharepoint mensioned in https://addbalance.com/word/MappedControls.htm.
The solution for this is simply explained https://sharepointmaven.com/how-to-add-sharepoint-metadata-to-word-documents-using-quick-parts-document-properties/ as well, which is:

1. create a SharePoint Team page
2. add a Documents Library
3. create a Word doc at the library (or upload an existing one as I did)
4. add new columns/properties to the Documents Library
5. these properties can now be inserted in Word with Quick Parts's Document Properties, keeping the mapping function
6. you can also download the file with those properties built-in or make some template from it

That's neat! Better and simpler than what I was expecting.