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.
Any recs for moving this into Anki? I already use Anki for cards I created while going through Genki with my tutor, and world capitals.