Order Catalog Item and tag to existing Request. It will attach to the existing Request and generate new RITM and then update variable

var childItem = 'sys_id of item';
var reqHelper = new GlideappCalculationHelper();
reqHelper.addItemToExistingRequest(current.request, childItem, 1);
reqHelper.rebalanceRequest(current.request);
//update/add values to variables on item  
var grReqItem = new GlideRecord('sc_req_item');
grReqItem.addQuery('request', current.request);
grReqItem.addNullQuery('parent');
grReqItem.addQuery('cat_item', childItem);
grReqItem.query();
if (grReqItem.next()) {
    grReqItem.variables.requested_for = 'user_sys_id';
    grReqItem.parent = current.sys_id;
    grReqItem.variables.access = true;
    grReqItem.update();
}