Sure, you could do this in a subclass and override advanceTime(). Remember advanceTime() is the only function in the IAnimatable interface.
You could subclass a Juggler, Tween or implement advanceTime() on a custom object, so if you wanted an adjustable modifier for time advancement, and wanted to also have only certain tweens or juggler groups slowed down or sped up by the same extra rate, you would simply override advanceTime() and multiply and pass the passedTime to the super based on 'half speed' 0.5x or 'double time' 2x
override public function advanceTime(time:Number):void
If you needed a whole group (or even all your animations) to be easily controlled by this modifier, then subclass a Juggler and add this juggler as a child of the main Starling juggler and add all your animations and tweens to your child juggler.
Maybe others have other suggestions, and I know others have mentioned other tween libraries have the 'slow mo' effects too, but the above might be one way to implement this with the Starling Juggler system...