Skush: Doesn't really make any sense imo.
A post marked as solution rewards 5 reputation points to the user that made it. While we currently view reputation as useless (and rightly so), back when it was implemented marking one's own post as a solution was seen as a method to abuse rep, thus why it isn't possible.
Staff members (not sure if all or some) can mark any post as a solution without awarding the reputation points, if you feel like going through the hassle of contacting them. Alternatively, ask for someone to link to your post and mark their post as a solution.
Since GOG is looking into changing the forum software, adding features to the current one is unlikely.