We generally disallow import of entities, since entities imported from other wikibase repositories would likely be using different IDs for items and properties. This may not only lead to dead links, but cause errors due to incompatible property definitions.
However, at least importing into a blank wiki (e.g. restoring from backup or setting up a test environment) should be allowed. An easy way to achieve this would be to introduce a setting that overrides the code in WikibaseHooks::onImportHandleRevisionXMLTag that prevents the import.