There is not a easy way I'm aware of but this how I have done number of times.
Define blind via(s), different but special sizes which will be used and documented for back drill.(say half thickness of the board and tied to GND net).
It'll give connectivity errors which can be ignored.
Provision of adding back drill information should be there in the PADS tool.
Now a days, Back drilling is used in most of the Designs and since PADS tool doesn't support
this, workaround solutions need to be used which are very tedious and frustrating.