to be more specific, should I post an answer to a question if I like at least one of the answers it already has, but I believe it is missing something that I myself could add, or should I just add said thing via a comment?
Two approaches, both valid:
1. Leave a comment on the answer you like.
Suggest the improvement. "Favorite" the question so it's easy for you to come back and check on later. Be prepared for the eventuality that the author may disagree, or may never respond.
2. Post your own answer.
If you think your additional content really adds something, the only way to guarantee its visibility is to post your own answer. In this situation be sure to point out how much of your answer is derivative of any others' work, and how your original contribution really makes yours the best answer. Then let the (long-term) voting sort it out.
Your (presumably) better answer may never overcome the benefit of early-posting that the other answers received. But cream will rise, and wise future readers will look further than just the accepted answer.