The X5 has tweeters mounted up around the side view mirrors. I suppose you could replace them, if you could find some of compatible size, but why?
There are a total of 8 speakers in the stock system, two on the dashboard near the windshield (mid-range), two near the side mirrors (tweeters), two in the front doors (low-mid freq) and two in the rear doors (same as front doors).
I would suspect it would be a major pain in the balls to get to the dash speakers and you have to remove the door panels to get to the door speakers (tweeters included). Then you would have to find speakers that would fit in the existing housings.
What are you looking to accomplish, as the sound isn't all that bad. I could see adding subs maybe but thats it.