Dissector using sip dissector

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Hello, I'm currently working with sip messages and i need to pass additional information with them ( I'm basicly parsing sip from hex and then i need to pass 2 more information both 1 byte long ) I thought of writing lua dissecor. I've manged to write this information in another packet on another port with my dissector but preferably i want to write dissector that parse sip and then parse additional information so i will have all info inside one packet. Is it possible without rewriting sip ? I've tried to get sip with get_dissector() method and then add it to tree of my dissector but then Im getting null value instead of sip.