Drilling anextrahole in a cast magnesium riser is going to really put a weak spot in the riser.I am a tool maker by trade and I probably wouldn't do it.

I have seen magnesium risers break in half without extra holes being added.

With that being said,you should have a competent machine shop look at it and see if the area you want to add a hole to would be thick enough to handle the stress after the hole would be installed.