I am not an expert in terms of SMF; however, I doubt that such add-on exists for it, as it would require additional data sets to be stored in the database which negatively affects its (already low) performances. An alternative solution could be that a number of trustworthy members are assigned to moderate a particular section that has topics that need sharing (for example, groups and businesses section), and that the leading members of such groups PM the moderator with the additions to the topic.