As an alternative to T143687: Allow setting target namespace in importDump.php , wikibase items can only be placed in one namespace on the target wiki.
The scenario is that is very common for wikibase installations to place items in a namespace named Item: that isnt namespace 0. It is described in the Extension page.
Importing Property pages from Wikidata into a wikibase installation occurs without problem, provided the target namespace name is Property. The namespace number can differ.
However, importing from source namespace 0 has no namespace name, so that doesnt provide the importer any way to 'guess' the correct target namespace is the Item: namespace.
However, there is typically/only one target namespace which can hold wikibase-item pages, so that could be used as a way to automatically map the pages from the source namespace 0 to the target namespace Item:.
This mapping is contained in $wgNamespaceContentModels.
I see Flow also uses $wgNamespaceContentModels and could benefit from this import feature.
There may be other extensions using this, but I cant find them in MediaWiki.org