Hello Wim,
Off the top of my head, I am not aware of a way to customize the Layer Stackup print; I think it's a pretty 'canned' output, but we'd have to research this a little to conrfirm. You'd need a current Support contract to do this, but I can only suggest submitting a Service Request so that one of us can research and confirm whether we can offer any suggestions here or whether it would need to be proposed as an enhancement request on the Mentor Ideas site.
I'm sorry not to be able to offer a more helpful answer straight off.
Kind regards,
John