Hey Jessica,
You'll need to implement some custom code to do this using the forms 2.0 API(http://developers.marketo.com/documentation/websites/forms-2-0/). It would look something like this:
MktoForms2.onValidate( function(form){
var vals = form.getValues();
if (vals.yourValue = thisValue){ //check the submitted values here
form.addHiddenFields({ "newField" : "newValue"}); //add conditional values here
} //iterate with 'else if' here if necessary
}
)