How to self-study Genki items?

You can’t customize WaniKani to give you kanji ahead of schedule, but you can use other SRS systems! I use Anki to review vocabulary for Minna no Nihongo. There are probably premade Genki Anki decks that you could find and download with a little searching. If Anki isn’t your style, there are other programs like Memrise and Kitsun.io that you could check out, though I don’t use either, so I can’t tell you how they integrate with Genki, and I don’t know how they’re priced (Anki is free).

And like @Killua099 said, you probably don’t have to memorize the kanji yet since you can rely on furigana until you learn the kanji in WK. However, that said, I have found it helpful with MNN to look up all of the kanji that I encounter and learn how to write them, even if I don’t attempt to memorize them yet. It helps me remember the vocab if I can connect the reading of the word to the kanji. I’ve found the Keisei Semantic-Phonetic Composition script to be super helpful for deciphering kanji that I hadn’t formally learned yet. I’ve been looking up all the MNN kanji in WK and looking at that script. Even though I’m not actively trying to memorize them early, it does make it a lot easier for me to learn the kanji in WK later down the line, so I don’t feel like that work is wasted.