To skip items, we can use the collective.transmogrifier.sections.condition section. The following example shows how to use this section, together with a catalog query.
[only_nonexisting_items]
blueprint = collective.transmogrifier.sections.condition
condition = python: len(modules['plone'].api.content.find(import_id=item['import_id'])) == 0
With this section in place, we skip all items, where we already have objects with the same import_id in the catalog.
