There's no API provided by CMIS that directly maps to the realm of workflows: all of the domain model and APIs are meant for document management.
That said, it's not impossible to establish some contracts between the BPM engine and your client application, so that workflow related informations (states, properties, workflow definitions, etc) will be mapped to the CMIS domain level entities, i.e. documents, properties, relationships and policies.
It's definitely not ideal, but as there's no direct workflow API in CMIS that's the only option you get.