//This is used to add the data source items.
$('#mlConsEmails').data("kendoMultiSelect").dataSource.add({ UserId: "EMP100XYZ", EmailId: "test@gmail.com" });
OR
var multi = $('#mlConsEmails').data('kendoMultiSelect');
//This is used for get the old values from multi select
kendo
var values = multi.value().slice();
//This is used for merge values in values object.
$.merge(values, "test22@gmail.com");
multi.value(values);
//This is used to refresh multi select kendo
multi.refresh();