In the above event, you can get the selected item text and use the dataSource of the multiselect to add new record with that text (using dataSoucre add method)
The implementation of this suggestion should look similar to the code snippet below:
select: function(e) {
var item = e.item;
var text = item.text();
var data = multiselect.dataSource;
data.add({text: text})
}
In addition, below is a dojo example, demonstrating the behavior: