Partially moving some emails addresses to 365 from existing domain but not all.

You can do a pilot with some of your users and email addresses, independent where your domain is hosted: https://support.office.com/en-us/article/domains-faq-1272bad0-4bd4-4796-8005-67d6fb3afc5a#bkmk_pilot

No hybrid required, if I understood your question correct.