This is done in the ContentBaseDefinition topic for each WikiBase.

The 'Import:' property should have a comma-separated list of namespaces to be imported.

For example:

 Import: Microsoft.Projects.Wiki,Microsoft.Projects.SomethingElse

When you import other namespaces, you get two effects:

1. as shown on the 'subscriptions' page, the 'inherited' RSS link will feed RSS data on these imported namespaces as well as this one.
2. TopicNames from one of the imported namespaces can be referenced without using a FullyQualifiedTopicName. If you have clashing TopicNames in any of the TopicNamespacesInScope, users will be presented with a drop-down list when clicking on the link.