You can use <span> tag to create textbox there. You need to create free format quesiotn and then put below code with radiobutton.
<TABLE width="100%"><tr><td><Span id="Other"></Span></td></tr></table>
now you need to call one function using onClick event and put this code there.
var a="Comments? (Please specify) <INPUT type=text id='Q3AWHY_A13' size='80' name='Q3AWHY_A13'>";
document.getElementById("Other").innerHTML=a;
This code is from ssi v7. So i am not pasting code according to 6.6 . You need to call using simple onclick event