use field extension to modify existing fields, is it possible?

I am follow the following link, I want to use javascript to modify the rendering of sharepoint created and modified datetime field.

https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/guidance/migrate-from-jslink-to-spfx-extensions

However, the element.xml in SharePoint/assets folder mentioned in the article seems to add a new custom field type, is it possible to do so for existing field, especially OOTB field? like the column formatting JSON does.