Entry points in the source code to extend grr

Thank you very much for your work on this great project. I am trying to intercept all GRRMessages about to be sent from the grr_response_server to a client, I have gone through the source code for some time now, but to be honest the classes relations are intimidating and confusing. Is there a way through the grr_api or even through contributing to the source code, through which I can intercept all GRRMessages about to be sent ? . My true objective is to intercept the GRRFlows requests before being sent to the client and modify the message carrying the request and add a token to it.