Hi Borut,
Theoretically, it is feasible to do a staged migration when you want to migrate users from Exchange 2007 to a new Office 365 plan. As far as I know, you either have to use the FIM Connector for Office 365 or you can use multiple Dirsync servers syncing to each unique tenant. The key is you cannot sync the same objects into the different tenants. You must create Dirsync filtering on each Dirsync server. This is because Hybrid and staged migration both need Dirsync.
Thanks,
Johnny Zhang