Hello,
I don't think this is relevant to the Kamailio implementation.
The Kamailio implementation doesn't do anything with headers like Cookie: at all. If an implementer of a Kamailio solution wants to do anything with the Cookie: header (or any other), they can do whatever they want using the Kamailio configuration file "programming language".
You can make use of all of the standard Kamailio header/parameter selects and transformations to help you do whatever you want with these headers.