I’m afraid it’s not possible. While metadata config values may be objects with nested properties, the grammar used to parse .items files doesn’t support it.
For those cases you would have to use the UI.
(the metadata is decoupled from the item itself so you can have the item in a file and the metadata for a certain namespace in the JSONDB as long as it’s not conflicting).
For this UI widgets-related metadata it’s not that far fetched to want to use the UI since you’ll get a live WYSIWYG preview.