there is an actual way to add a push button for engine start, the process is very involved and was documented for an E39, and i am sure, it can be done for E53...

I personally find engine push start totally pointless unless you have that option when the key can stay in your pocket - i drove cars when i had to insert a card into the keyslot, and the next step was to push button - why? i could crank the key while it was in my hands...

with convenience option, the push button makes sense - i get in the car, and push the button - the key stay in my pocket...

just my $0.02