Have you seen this? http://docs.coronalabs.com/daily/api/library/widget/newSwitch.html
I haven’t used the switch control yet, so I’m not the best person to talk about it. But it looks like you would simply need to specify the spritesheet with the images that you need.
I don’t think you really want a mask… I think you want to tailor the whole set of images for your switch. Remember: A mask is used to hide certain parts of the display object - you’re not trying to do that; You’re trying to change the look of the switch as a whole.
If you do want to mask your switch, you could always put it into a display group and apply the mask to the group. I suppose you could even apply a mask to the switch object itself, as it will be a display object like any other.