How can I reuse the output of an XML service as input for a new one?

You can't (at least not with vanilla ChangeMan ZMF) ... unless you have AbitMORE SCM Commander (ASC for short) available ...

With ASC, you can store the result of an XML service (typically an XML LIST service) as a so called basket. A basket is a kind of CSV file with 1 header record, followed by 1 record for each result of the XML list service. Each such record contains the values for each of the requested result tags. Here is a sample of such basket:

That basket can then be used as input (request tags) for a subsequent XML service (e.g. to apply an XML DELETE, UPDATE or CREATE service).

What is more, when you created a basket, ASC will help you in identifying all possible XML services that can be applied using each of the tags in the basket as an input tag for the subsequent XML service. That itself is a huge saving in searching for the candidate XML services that can be applied to a basket!