generate single XML schema from multiple packages

[Caller171]

Is it possible to generate a single XML schema from multiple packages?
We wish to break a rather complex XML structure into separate smaller packages, but still be able to generate a single XML schema.

The documentation states:

A schema element is generated for the target package. If the target package includes classes from another package, which has the tagged values "targetNamespace" and "targetNamespacePrefix" set, these are included as attributes of the schema element.

In addition, an import or include element is created for each referenced package. (An include element is used if the external package shares the same targetNamespace tagged value as the target package. An import element is used where the targetNamespaces differ).

How can a target package include classes from another package?

The "Generate Xml Schema" dialog does not contain the checkbox "Include all Child Packages", which "Generate Source Code" has.

Thanks,
Caller