As far as I am aware, this cannot be user defined. But the devs are open to suggestions. So if you need a specific new item type, post here and explain why you need the specific item type, how it is not covered by already existing item types, and which combinations of fields you would need to make that item type work.

I guess there is a consensus that new item types are only considered, if there is absolutely no way to make them work within an already exisitng item type.

Note that you can also make use of the "extra" field to add information that cannot be added in exisiting item types. see here:
https://www.zotero.org/support/kb/item_types_and_fields#citing_fields_from_extra

You can search and you will find a number of discussions for new item types over the years, for example here: https://forums.zotero.org/discussion/comment/367373 or here: https://forums.zotero.org/discussion/comment/423274