Hi!
There is no automation operation that get versions of one document (maybe it should).
Anyway, you can get away with a run script operation
Here is an example of script that get all published versions of a document (proxies) and remove them :
proxies = Session.getProxies(new org.nuxeo.ecm.core.api.IdRef(Document.id),null);
for(proxy : proxies)
{ Session.removeDocument(proxy.getRef()); }
In your case, you are not looking for proxies but versions, so it should be something like :
version = Session.GetVersions(....
For more informations of what methods are available with the object Session, you can take a look at:
CoreSession
You will see that they are several methods to get the versions of a document depending on what you plan to do with it (just need the refs or all document models).
Fred
