Apply site script to existing site?

Hi,

I feel I am being really stupid. I have a JSON script that adds a new Document library and various fields and joins a hubsite. However I want to add a field to an existing list during site creation.

Various commands in the schema (https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/site-design-json-schema) say that you can add them to existing lists. I have created a site list but how do I reference the existing list to add it?

Hopefully that makes sense