in my scenario, I want to have some services to be fixed (as in not needing to be updated) and as time goes by adding other services. (I'm using one DB instance, but it shouldn't matter in service broker)
I want to set up the fixed ones in a way to be able to send back a message to the initiator of any message in its queue without me changing its logic and procedures every time I add another service.
is it even possible or do I have to add more logic as new services are created?
If I'm understanding your question correctly, this is how Service Broker works by default. Which is to say that a conversation is between two parties (initiator and target). Once that conversation is established, either party can send messages on it and they will go to the other party. So, if you want to send a message back to the initiator, just send a message on the same conversation handle as the message was received on and you should be good to go.