As far as I’m aware there is no plans to add them, because the system is built around 1 character = 1 keyword + 1 pinyin spelling. Since 1 character often maps to multiple meanings and/or pronunciations, how do you add the words that use the other meanings/pronunciations? This is the reason why 还 is taught as huán not hái. So unless they relax their own rules, you can’t add a bunch of words into the base curriculum because they map to different meanings (same with say 乐 being in both 快乐 and 音乐 - you can’t have both while only mapping 乐 to a single pinyin+meaning).
So this you can do for multi-character workds - you add them manually by turning on dictionary items. See here: Introduction to dictionary items | HanziHero Docs. So I have 还是 in my queue despite it not being an official HH word.