I think the only answer is: You earn the requisite reputation, then post the comment.
Users who flout this convention (by posting a comment as an answer or editing existing posts) will usually have their posts flagged or edits declined. Site moderators must then take the time to process said flags. This is not an efficient use of other's time.
Requests to lower the comment-rep-requirement have been declined by the StackExchange team. See this meta.SO question as an example: Lower the amount of reputation needed to comment
Here is a full list of reputation privileges -- 50 rep is not a terribly difficult milestone to reach. Even just asking questions earns you rep.