Alright, I hear a lot of people (rightfully) say that some things are impossible due to the protocol,
But there are always people absolutely sure that it is possible...
I somehow doubt those people even know what the protocol is, and therefor I think it needs some clearing up...
As humans, we can communicate because we both know english. if I say dog, you know what I'm talking about, if I say cat, you know what I'm talking about.
However, when one of us suddenly starts using spanish words, you would not be able to understand me anymore, unless also you spoke spanish...
The same story applies for Messenger, the protocol is like a language it uses to commicate with other Messengers, and you cannot add new things unless it is somehow defined in the protocol. and you CAN add new stuff, but then other messengers you are communicating with would need that same stuff too, else they would no longer be able to work together. So you can add new stuff like display pictures for different contacts, but your contacts would need to install the same plugin/program, and contacts not doing that would not even see your DP.
I hope this cleared up a bit,
if you have any more questions, please do not hesitate to ask