The jBPM engine inside Alfresco is gifted with knowledge of the resources in the Alfresco environment. These include the document resources, people resources (IDs and groups), and other work flows. Among other things, this allows a fairly simple interface from the Javascript steps you can embed in the work flow steps to access and modify the Alfresco repository.
You could certainly arrange that other systems could use the Alfresco jBPM, presuming that you had shared credentials, via web scripts or web services. Those other systems could have bi-directional connections to allow for status displays, notifications and remote triggering.
You would still have to have jBPM inside of Alfresco because Alfresco depends on it for many actions, such as repeating events, like nightly purging. It might be possible to write a set of shims to hand off any more complex actions to another jBPM server. The jBPM server would then have to have a set of adapters available to access document properties, etc.