No, it’s not possible to add your own material to Wanikani.
Have you tried checking Kitsun? It’s a SRS website being built by the level 60 WK user @neicul. It incorporates a lot of fresh new ideas that you can see on WK scripts and it’s as user-friendly as Wanikani.
Here’s the forum thread:
The screenshots on the main post aren’t up to date, since a few days ago there was a huge update with a lot of new features + redesign. To see how it looks now, check this post: