I see from the original posters profile that he has an E320, not sure what year, but I'm guessing a 98 or newer with the "smartkey".
S500, I think what he means is the thin valet key that the smartkey cars came with for the first 3 or 4 years, it has a small slide out blade key, kind of different than what the full function smartkey has.
YES, I believe that you can unlock the door with the blade, then put the valet smartkey into the ignition switch and the alarm will go off, possibly not until you turn the key in the ignition though.
Just try it out if you need to. Roll down the drivers window, get out and lock the car with a remote key. Wait 10-15 seconds, then pull up the door lock knob, so the alarm goes off. Then unlock with the blade key and try disarming the alarm by inserting/turning the key. Post your results, then we'll all know for sure.