Not clear based on your question
Are the mounting holes electrical symbols in your schematic (they have at least one pin connected to a net)?
If yes, you should be able to route to the pin. Make sure your symbol calls out the correct padstack.
If no, you can make the mounting holes "vias" by going into the padstack editor and making the padstack of any size you want -- then add that "via" to the via list and click on the top plane and add via at the proper locations -- choose your new via when routing.