I wouldn't do any drilling of the bell of a metal stick blender. For a variety of reasons. One: my own personal safety. Two: potentially ruining my stick blender arm. Three: potentially ruining my drill Four: potentially producing rough, sharp edges in the bell of my stick blender arm that would be a danger when cleaning (again, personal safety)
What I would do, is go online and find out if I can order a replacement blender arm for my brand of stick blender that already has cut-out holes. My stick blender has a couple of different options for replacement arms, as do many other brands. If I couldn't find one online for my particular SB, I'd go buy a new one. They're pretty cheap compared to the damage I know I would cause if I tried to drill a hole into a stainless steel SB arm.