I was trying to figure it out to submit a comma delimited value on a multiple select. Here's my sample code:
$( 'myform').ajaxForm({
    beforeSubmit: function(data) {
        var queryString = compressParam($.param(data));
        //console.log('params:'+queryString);
    },
    success: function(response){
        $('div').html(response);
    }
    error: function(a,e,et) { alert('ERROR 101: '+et); }
});
function compressParam(data) {
    data = data.replace(/([^&=]+=)([^&]*)(.*?)&\1([^&]*)/g, "$1$2,$4$3");
    return /([^&=]+=).*?&\1/.test(data) ? compressParam(data) : data;
}
I found a function that will combine same parameter. Works like a charm but don't know how to submit as data or do you have other idea how to submit a multiple parameter in comma delimited using ajaxForm?