unfortunately I don't think you can do this... I don't understand why Apple decides to limit the user in this way, but they do. In order to rearrange them, you'd have to just put a number at the beginning of each list or something similar to change the order.
I know you didn't ask for alternatives, but have you ever checked out Adium? I much prefer it over iChat, but handling file transfers is still finicky and there's no video support, so I only use iChat when I need these features.