Well, you can always edit your initial post. If you're creating a post and you know you want to add something as the top reply later, many people will comment something like "reserved" and edit that reply later. You'll see this in the creative sections when people are sharing writing projects.
Can I ask what you were hoping to use a feature like that for? Just curious, but it's good to know how people want to use the forums and what we could potentially improve on.