I'm not sure if it'd work...but if it was me, I'd just grind out the opening. You can always return it to stock later & nobody would care (or even know). A purist would be uptight, so I'd only do it on a higher-mileage example. If you have less than 150k miles, I'd stick with stock and get the correct 5 1/2" size. Collectors are aware of this car...if the current economy improves I suspect they'll bump up considerably in value.
Option 1 requires cutting or fab work. An extra 1/2" diameter is a problem. I tried the same 5 3/4" thing on the rear deck - the speakers are better than OEM but it required a lot of cutting and the speakers have to sit pretty high.
Option 2 is to use a spacer & a grille. That way you won't need any sheet metal work. The hood latch will likely be a problem, might need a spacer. Washers would likely do the trick. If you push the speaker cone up out of the opening it'll fit.
Option 3 is upgrading the stock speakers to the BMW Premium housings with better aftermarket stuff. It's the most expensive but ultimately the best. It requires no fabrication, improves collector value, and allows you to design the system a bit better.
This car sucks for audio IMHO. If I was really into music I'd design a better mousetrap. Upgraded speakers, better amp. Stock tweeter pods in front and another pair in the rear c-pillars, mid-ranges in the rear side panels and center console, woofer box through the center of the rear seat.