I know this is an old thread but thought it would be useful to let people know I have managed to retrofit the same radio (9246503) into my 2009 E89 Z4. I used the E89 DAB antenna (9123995) which connects to the radio via a fakra cable. The 318 Touring would use a different antenna and amplifier on the right side rear window but you might be able to use a window antenna.
In terms of coding you might need to add 654 to the coding on the CAS and FRM modules to enable DAB if it isn't already enabled.
I hope that helps someone
Thanks,
Nick