Can I modify an existing doctype from a new app?

We wan to create a custom workflow for Purchase Receipt. One of the recommendations I repeatedly hear in the forum is to create new app for modifications to handle future updates. How do I modify the existing doctype from a new app? Do I need to make a copy of the entire module, and make the changes I need? Or, just make the modifications I need - but how will I integrate the changes to the Buying module?

https://kb.frappe.io/kb/app-development/how-to-add-customizations-to-app