Thats a genuine Combox you showed up above. I wonder why BMW aren't showing you the retro kit, did you see the link I posted? I wonder if it's to do with the lack of pre-wiring.
Anyway, maybe check out the E90 forums (thats where I've done most of my research), as the Z4 E89 is based on the E90 2000BN electrical system from what I understand.
Essentially even if BMW don't have an official retrofit kit for you, you can still retrofit the original OEM parts using spares, if you order the right ones. BimmerNav/BimmerTech just package up the spares for you with some harnesses that are hard to get from BMW or out of manufacture. A LOT of 3-series owners seem to have done this.
I can find a few people that have said they've done it in a Z4 E89 but can't find anyone that has pictures or documentation. Essentially no-one has done it usefully in an E89 Z4. Maybe I'll be the first. Am definitely going to do it though.