Comment by domenicd

I've thought about this problem myself a lot. I don't have a great answer.

The easiest thing I can think of is just grab https://ankiweb.net/shared/info/369908962 , and schedule everything you've already seen in WaniKani for immediate review, heavily relying on pressing easy to try to get things into a longer interval if you already feel you know them. This probably works best if you're relatively low-level on WaniKani. You might be able to use the WaniKani and Anki APIs to automate this to some extent.

Googling around, it seems others have run into this, e.g. https://community.wanikani.com/t/export-wanikani-srs-progres... and https://community.wanikani.com/t/userscript-wani-kani-export... look promising. https://www.reddit.com/r/LearnJapanese/comments/n1pihp/why_y... also seems good.