No you can"t add keyless go after manufacture. This is why I got such an amazing deal on my car. I test drove car and asked about push start/keyless entry, dealer told me they install push button after purchase, because cars on the lot could be stolen. Negotiated deal, signed paperwork, went to car and dealer spent 30 min trying to install the button.....turns out car was not equipped. He said it would cost thousands to install after manufacture as it runs through the engine. Needless to say, I told him he sold me a car with keyless go, he agreed. We cancelled the deal, he searched and searched, ultimately the only "matching" car with keyless go had every option and the dealer ended up giving it to me for the same price to make it right.
I still find it amazing that a Mercedes did not come standard with keyless. Hear it is now standard on the 2016.