You would have to locate the code within your Email that collects the user input for your Yes/No field. The textbox should look something like this.
Code:
<form>
First name:< input type="text" name="firstname"><br>
Last name:< input type="text" name="lastname">
< /form>
The "form" tags should be several lines away, depending on where the field is within your email.
You would need to change the input type from "text" to the type you want. A "checkbox" may be easier than a "radio" because radio buttons use Groups. The above code uses "name". You should leave this the same so the form knows which field in the database to associate with the user input field. There may be an ID also, leave it the same.
Here is an example
http://www.w3schools.com/html/html_forms.asp