Does Pentaho support asyncronous workflow?

Specifically what I need is to be able to:

1. define a long-running workflow in XPDL (or the like)
2. the workflow engine would run the workflow and for each workflow item call a custom web service I wrote (in REST, XMLRPC, or SOAP) outside of Pentaho
3. once that particular web service is finished it would call the next, etc., but the issue is that each one will probably last a few hours and the next web service must not run until the first is finished. Most web services will probably timeout before then unless they are asyncronous.
4. and have Pentaho execute, and track it for me.

This is for a custom ETL/data warehouse setup.

Can Pentaho do that?