It is already possible to export and import Wikibase items via the main export and importing mechanism provided by MediaWiki. This process retains the IDs. I have created a little guide at
https://wikibase.consulting/transferring-wikibase-data-between-wikis/. I did not address creation of new items/properties after such an import yet, so beware that an extra step might be needed.
I agree with you that it is useful to retain IDs. My company is helping the German National Library with importing their GND dataset into Wikibase. ID stability is important in this project. What's more, since the GND already has IDs, ideally these would be used, rather than new Q-IDs. IDs in different formats are not supported by Wikibase yet, but hopefully it is not too difficult to implement, as it seems rather useful for many projects. I'll be looking into this soon.
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