You can turn a field into a multi-selection this way:
1) Make your field a string field with some length (512 chars)
2) Invoke the widget settings dialog for the field and delete the existing widget, set it to Composite.Widgets.String.DataIdMultiSelector and select your data type.
This approach will give you a string field on your datatype that contains a comma separated list. If you choose the 'compact' selection type this list is ordered by the way.