With the Charlemagne expansion you can form your own kingdoms and eventually empires if you have enough territory, titles and prestige to do it. So I founded my own kingdom and later my own empire, but I, of course, have no de jure territory for either (some wiki's suggest that whatever you hold at the time becomes de jure, but that doesn't seem to be the case). Can territory eventually drift into my custom empire? And related, it seems like since I have no de jure territory, my crown laws apply to absolutely nobody. I set my crown authority to medium to stop my vassals fighting among themselves, but it seems to make no difference. They seem to be working on the crown laws of the kingdoms they belong to instead of mine.
When you create a custom kingdom or empire, any territory you control that could drift into it will immediately become a de jure part of it. This means that any duchies you completely control will become part of a new kingdom and any kingdoms you completely control will become part of a new empire.
Once you have a custom kingdom or empire, duchies or kingdoms respectively can drift into it. This happens in the same way as with any other kingdom or empire.