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.  Flags require a site moderator to take the time to process the flag -- it is not a good use of 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](http://meta.stackoverflow.com/questions/12119/lower-the-amount-of-reputation-needed-to-comment)

Here is a full [list of reputation privileged](http://serverfault.com/privileges) -- 50 rep is not a terribly difficult milestone to reach.  Even just *asking* questions earns you rep.