I've added beads to shotguns. I just had a look at the Stoeger Grand gun on their website.

Generally speaking, this is a job for a gunsmith. The reason why is that unless you have a mill or a milling vise and a drill press, you're probably not going to get the hole for the second bead centered on the rib. I'd be doing this job on my mill just to get the bead mounting hold into the center of the rib, down to the thousandth. Next, you have to tap the hole. There are several different tap specs for beads - some metric, some English. 3-56 is a common English tap - but I'll wager you won't have this tap in your tool set.

Lastly, there is a question of how you want your "close" bead to be set up. Some double-bead shotguns require that you "balance" the front bead on top of the rear bead when you're sighting. Others will have a bead that is smaller than the front bead as you're looking down the rib.

If you are the one drilling and tapping the hole for the bead, you must be certain that you're not penetrating the barrel wall. This is another area where having the correct tool is important - I can set the depth stop on a mill or drill press to prevent over-penetration of the hole. 

Oh, and BTW: for small diameter, fine-threaded screws, remember that drill bits rarely, very, very rarely, drill a hole of the size they claim. They're almost invariably a couple thousandths of an inch over-sized. This doesn't matter when you're drilling/tapping holes for #8 screws and larger, but for small screws (#4 and smaller) with fine (40 TPI or more) threads, this becomes an issue in that you get a "sloppy" engagement of the threads.