You can't do it with addterm I'm afraid. Addterm intentions cannot add things into subsearches. Which leaves
a) the 'stringreplace' intention, and you can find some examples of this usage in the UI Examples app.
b) take a look at Sideview Utils which does away with the whole idea of intentions and allows you to plug form element values directly into searches as straight $foo$ tokens. (Full disclosure: i am the creator of Sideview Utils)