Hi,
I don't think that's possible as the touch triggers the onclick which is handled by shadowbox and the shadowbox code is on our server.
If you switch to shadowbox (without the external), then it will use the shadowbox that you have on your website.
So you can copy our shadowbox file on your website, and add it in your template and then you can modify it and potentially be able to change that so that it skips the onclick process on mobile and instead register itself on the touch trigger.
So it's technically possible, but it will require proper development.
I would also like to mention this plugin:
www.hikashop.com/marketplace/product/117...pups-by-obsidev.html
It adds additional image popup modes which might be better suited to your needs.