Liesbeth,
Nested workflows or workflow interdependencies are not directly supported by the package, but you can of course hack it in by using callbacks which trigger related workflow events.
You'll see some examples of this in my prototye peer review module:
http://amped.dnsalias.com:8000/peer-review.tgz
In it, I model the document to be reviewed as a single workflow, and each individual reviewer solicitation/confirmation/submission cycle as a sub-workflow.
The code won't run, but perhaps my callbacks might help you.
