A Realm database is just a (compact) file that you can send as you see fit. As most BaaS solutions has their own local storage and API it may not always make sense to integrate, except that you could easily store a small Realm file as a blob in any BaaS and transfer it remotely.

It will also really soon be possible to export some or all objects as json which you then again can send to remote services. That does however take up much more space/bandwidth compared to Realms compact data format, but could be needed if you want to persist remotely in another database format.