Well if the stock 18's are 8.5inch wide then yea you should be able to do it.
I don't think you can stick 17inch rubber on an 18inch wheel if that's what you mean. But hey I could be wrong.
ZHP Mods: JB Lightweight fly wheel (10.5 pounds), Koni FSD ProKit, Supreme Power Drop Hats, Hotchkis front(30.2mm) and rear( 25.4mm) sway bars, Sprint Booster, AFE Stage 1 Intake (Dry Filter), Morimoto EvoX-R Headlights, Orion V4 Angel Eyes.
330xi Mods: Front Koni Yellow Struts and Rob43 Rear 9 Way adjustable Rear Dampers, H&R Sport Springs, Supreme Power Drop Hats, ZHP 5 speed shift knob