Sorry. It's not possible using internal miniBB tools. Also, even manually it could not be impossible to achieve.
All messages in miniBB have unique numerical auto-incremental IDs assigned to them, and in the thread, they are sorted by these IDs in ascending order. Thus messages in the thread do not have +1 increasing IDs. For example, if you look at this thread, your message ID is 37842, mine is 37844 (this is visible if you mouse over the # link). It is because I replied in the other topic, and there my message's ID was 37843.
So if we would like to move the message up or down within the topic itself, that would mean, we would need to change its ID, so it fits the IDs above or below properly. In most cases, it's not possible, because IDs will be reserved by messages in the other topics. Also, changing the unique / auto-incremental ID manually means truly bad approach, it breaks the core rule of DB's architecture.