That looks like a new question to me. They can refer to the old one and add their expansion. They should not edit a new additional question into an old one, they should be encouraged to ask their own.
In the case that someone has a clarification they have two options, ask their clarified question as a new question, or do something else and ask it after they've amassed the relatively trivial 20 rep it takes to earn the comment privilege.
Adding an answer with the intent for it to be converted to a comment is suboptimal behavior and should not be encouraged, though it is an option for moderators when it does happen (typically moderators are advised not to do this for low rep users so as not to encourage it, though they typically do the conversion anyways).