Currently it doesn't seem possible to run sub-workflows within a 'parent’ workflow. I can be very helpfull when this is possible in terms of re-useability of parts of a workflow.
For instance, when you want to have different types of documents to be approved. The data validations of each document might be the same but each type of document follows its own workflow because of very different steps, it would be very helpfull to re-use a sub-workflow for the data validations for each of those parent workflows.