The bookmark importer in Opera Blink is a one time feature and doesnt allow adding new bookmarks afaik.
Not certain myself, but I know that if it will allow you to add new bookmarks they will not be in a folder. You can't re-import the folder (at least, without deleting the old one first) and if you import any bookmarks singly they appear in the QAB directly and not in a folder (so you'd still have to drag them to the correct folder). If space on your QAB is already tight, @pesala probably has the best suggestion.