You can add any words as kana only on the legacy site, by using the “All Results” button (which shows up even when there is only one entry matching). Right now the All Results buttons isn’t showing up on 2.0 when there’s only one entry available, so it makes it impossible to add the kana only entry. I recommend using the legacy site to edit the lists in the meantime, or at least when you’re wanting to create new kana-only entries!
I’ve brought up an idea with the team to make studying kana-only items easier. Right now you can toggle the green W mark while editing a list to disable/enable the writing prompt for just that word itself, however this won’t make it so you write kana only and instead completely disables the writing prompt, and reading and definition prompts would still show the writing using kanji for the word itself. A cool idea might be when you disable the W, that it asks if you’d like to study the item as kana-only, so you wouldn’t have to create a new item, and it would automatically convert it to kana when you’re studying any prompt type. It’s something we should think about! This would also mean you could switch the kanji writing back on when the book introduces kanji for the word, and it wouldn’t double up the reviews for having two individual entries for kanji and kana only, and would automatically convert your kana only item to the version using kanji. Since it’d be the same item, the scheduling would also need to be reset for the item if you flipped kanji writing back on, so it’s not grabbing the review data from when you would have kana only on. (So it would be treated as a new item).
The other alternative would be to have 2.0 match legacy’s behavior, where the All Results button shows up for every word regardless of how many entries exist, so you would be able to create kana only entries. This will be implemented at the very least, but I think the first idea would be a much better improvement.