It's actually pretty easy, even if it didn't "Come" with keyless entry.
All you need to do is remove the lower dash cover beneath the wheel. Then the metal plate behind that, and the lower & upper HVAC ducts. There are 4 screws that hold the BCM to the dash. Remove them, and all the plugs on the back of the BCM, to be able to pull it out. 3 screws connect the RKIM to the BCM assembly. Pull them, pull the assembly. Replace it with one from a junkyard car that you grabbed the fob from.
I did it on both of my 300Ms now. Both have working keyless.