Howto add a pulldown menu to Thunderbird?

[oebilgen]

Hi list,

I want add a pulldown menu to Thunderbird. I want that Thunderbird reads the value and adds it to the mail header as a line. For example if the select tomato, I want see "I like: tomato" in the header of the mail. Is it possible? Should I change the hardcode or is Thunderbird that flexible? This is something personal and not interested with the current development of Thunderbird, but is still very very important.

Thanks in advance,