Grouped Values Dropdown: allowing multiple selection in the widget

Is it possible to alter this widget so that you can select more than one response - effectively combine this feature with that of the asmSelect widget?

Unfortunately, it's not possible. I've forwarded this to our widget team as a feature request. If there are updates, you should know it through this thread.

For the meantime, you may consider using the asmSelect widget instead or the following as a workaround:

http://widgets.jotform.com/widget/checkbox_in_dropdown 

http://widgets.jotform.com/widget/line_checkbox