if several values from other table can be related to Order, maybe you need to use MultiSelect control.
However Lightbox controls seem only to be able to mapTo fields from Order table.
That is correct, the controls checks the mapped property of the data object. If you store the relation in a separate table - [OrderId,SomethingAnotherId], the data item that is loaded to the client-side still has to contain it. So you have to create appropriate property in the order model class and handle it when you loading and saving the data.
If you add multiselect control, the mapped property can contain comma-separated values:
EventModel.MappedProperty = string.Join(",", AnotherIds);