NEVER EVER modify the structure of your helmet unless you know how to properly re-enforce the EPS. And even then it is not a good thing to do because the liner will most likely crack, splinter, and break down faster. The EPS is designed to distribute the force during an impact. And if you cut a nice little hole into it, you just allowed that to fail.
Pci or parker pumper can install the fitting for you @ a reasonable price. That way you're sure its in the right spot.. I drilled mine for the "hans mounts", using the directions.. If it didn't have the exact location, i would have had it done by someone who knows exactly where to put them..