It's not possible to make a topic self-moderated if you didn't select "self moderated" when creating the topic. That's how it should be and I don't agree with allowing users to do so.
The poster has the right to know whether his/her post can be deleted by the topic starter or not.
Assume that a topic isn't self moderated and I make a post. (I may not want to post in a thread which is self-moderated). Then the topic starter makes the topic self-moderatod and deleted my posts.