I’ve contributed a migration page provider to JSPWiki. The engine supports a variety of page providers that handle the storage and retrieval of page contents from whatever store you want to use. This provider is really just a proxy for some real provider with the key feature that if the real provider is empty (no pages), the migration provider will populate the real provider with content from yet another provider.
The idea was pulled out of the original JDBC page provider which has a similar feature where it will populate itself from another provider. I made it into a separate provider so I can go the other way if necessary. That is, if my JDBC provider proves to be the source of my local wiki crashes, I can migrate the content to a standard page provider.
There is one main shortcoming: the modification dates get lost due to the design of the page provider API (adding a new page to a provider always gives it the current date).