I have not been able to find where to add a bid modifier to the association between an ad group and a user list via the UI, however per the link I provided you can programmatically set a bid modifier on the association.
> As with other types of criteria, you can assign other properties to the AdGroupCriterion object, such as bid overrides.
Is there any way to achieve the desired behavior without creating multiple ad groups? (one for the broad audience and one with a modified bid for the user list)