In order to save multiple values using a Picker Control in SmartForms, the "Save as XML" option must be selected in the properties pane. This option saves the values as a delimiter, which requires a backend SmartObject that supports delimiters. For more information on this topic, please see the Save As XML section in the K2 SmartForms User Guide link below.
--> K2 SmartForms User Guide: Picker Control