Identity 3 custom passwordhasher

I can't quite seem to piece together how to get a custom passwordhasher working in Identity 3 with ASP.net 5/MVC6. I have an existing user database which had it's own mechanism for password hashing, which is why I need to replace the built in one (rather than reset everyone's password ;))

This was somewhat easy in Identity 2, but I'm scratching my head with 3.

Thanks for any assistance!