I would like to study kanji on renshuu as a reinforcement of those I'm learning on Wanikani. Is it possible to display kanji in "Starter Kanji" the same order as in Wanikani? Or to synchronyze them with Api key?
You may want to check out in the Lesson Center for Kanji, the Lesson: WaniKani (74 lessons)By: 高「たか」
It breaks out the Kanji corresponding to the 60 Wani Kani Levels (the extra levels are for Kanji that were added in later)
So, if you schedule those lessons when you reach them in Wani Kani, it should do what you want.
As a side note, one of the proposed Renshuu projects in Kao Club that may be of interest to you is to import Wani Kani mastery levels (as of writing this, it has not been approved yet).