I think what you want can't be done. In the dropdown you're able to select only one value. If your application required the selection of multiple values, my suggestion would be to create a seperate object and link it with a many to many association. You configure the values into a new attribute (either string, or the enumaration you already have) and use a reference set selector. This allows you to select multiple values, though not in a dropdown. Hope this helps you in the right direction.
I don't think so you can select multiple values using dropdown instead you can use multi select widget from app store to achieve multi functionality.
Hope this helps!