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

Yes, this is called Hybrid setup. You can have e-mail boxes on-prem or on Office 365 at the same time. So you can move half your users over and not the other half. Or one department for awhile then the rest. It's really up to you. And you can even move the e-mail back to On-Premise if you want later on.

Here is the documentation for it all: https://technet.microsoft.com/library/jj200581(v=exchg.160).aspx