If you have the N20 motor, then theres nothing you can do to make it sound like an //M car. At best you could do a cold air intake and maybe find an aftermarket exhaust( theres none that i know of... maybe a custom job?). If it has the N55 motor then you could make it sound better but that still would sound like a 135i, 335i, etc. with an aftermarket exhaust (i think some member on this forum got a 135i performance exhaust to work but i dont remember). If you really tried maybe you could get it to sound kinda close to a F87 or F80/F82.
No, it will never get close to the sound of a S85, S65, S62, etc. if you /really/ wanted to have a BMW that sounded like an, as you put it, "real M series" then you need an actual //M car. The "m-sport' package on the X1 is little more than a pack of stickers and an aero-kit.
Here is a link of someone who did a custom exhaust for their X1 28i... although i find the quad exhausts to be in poor taste and sacrilegious, that is a matter of taste: