I forgot to add an edit summary when I posted an edition to my question. Can I add it afterwards?
It is not possible to change or to add to the edit summary after the revision is completed. Note however that edits made within 5 minutes by the same author (if nobody else interferes) will be put into a unique revision.
Thus if you click "save" and note the omission right away, then you can make your addition by restarting the "edit."
Moderators could change the edit summary. But this is intended for drastic cases, say, somebody includes insults in the edit summary. A missing edit summary is usually not a big deal; if it were considered as such it would be software-enforced (it is like this for suggested edits). Thus, do not bother moderators with the request to add your summary.