There’s maybe some confusion about the terminology. There’s a strict definition for radical and a loose definition for radical.
The 214 radicals are the strict definition. Each kanji only has one radical. They are used for cataloging kanji and determine how kanji are ordered in a kanji dictionary. The Japanese government gives schools the information they need to teach to kids on kanji and this is one element that gets officially defined.
The WK radicals fall under a loose definition. These are more like the parts that make up a kanji and have no official standing or categorization.
If you want a tool that converts kanji to WK’s radicals, you need to use something related to WK, because no one else will know what you’re talking about.
Why exactly do you want a “kanji to radical converter”? The site jisho.org has kanji parts info on kanji detail pages, but they might be unrelated to how WK does things.
[Edit: and of course you can also look at the WK detail page of a kanji to see which WK radicals it is made of.]
The strict definition@Leebo is talking about is that a kanji has exactly one radical. Wanikani “radicals” are all parts that make up a kanji, it’s like a decomposition of all parts, and some “WK radicals” never appear as “real radicals”. There are 478 WK radicals, but only 214 official ones.
Maybe you should get a few WK levels first before you worry about real radicals, they are not that essential in the beginning.
If you search by kanji, the map will show you its parts/radicals on the left side and related kanji on the right side of your kanji.
Of course, their parts/radicals do not correspond to WK radicals 1:1.
In RTK this is why they are called “Primitives”. The term “Radical” to mean “Any Component part of a Kanji” is a relatively new usage anyway. I suppose it would be less confusing if the difference between “Radical” and “部首” was explained.