We often get questions about how to configure BPM to use multiple identity providers (directories, database, etc.) so that users of BPM Workspace (for example) can be authenticated against different providers.
A little known fact about BPM is that it includes identity virtualization support through libOVD. It is not necessary to install a full blown OVD or to set up custom DB authenticators, etc. You can just use libOVD.
My colleague Christopher Karl Chan from Denmark has a good introduction to this topic here which I would encourage you to read if you are looking at this kind of requirement.