BuddyPress.org

Hi everybody,
I’m asking here the best way to alter/extend the private message system of buddypress. I suppose I have to develop a plugin but I don’t know what is the best way to design the plugin in a flexible way.

I need to extend the private message system of buddypress in this way:
I want to let a special kind of user (say the MODERATOR) to assign a priority to every private message sent to another kind of user (say the EXPERT). It is a way to not disturb the EXPERT everytime he receive a private message. The MODERATOR should act like a pre-filter, and approve/not approve the private messages sent to the EXPERT (who is very busy and cannot answer to silly private messages). Only when the MODERATOR approve the private message sent to the EXPERT, the EXPERT will see that particular message to his inbox. In practice I need supplementary information for each private message (like approved/not approved and so on).

Can someone please suggest me a good starting point to do this?
Thanks in advance.