Interesting idea, maybe I'll PM a moderator to ask if it's possible and an appropriate solution. Or a moderator can reply here if it's possible.
I just checked (in a section where self-moderated topics are enabled) and we have no option to change a topic to self-moderated.
And it's not bad in my opinion. We will see this kind of topic : "Oh no, the evil mod didn't want to put my topic in self-moderated to protect his corrupted friends."
Or : "Oh no, the evil mod changed the topic to self-moderated so that my opinion is deleted now."