As far as I'm aware the only way it can be done is via a chap at a garage called "prestige german cars", they have two outlets- main garage at south shields and secondary unit across the river at north shields. The guy that owns it is an exBMW mechanic. It's just down the road from me, and a group from the forum (from all over the country) had it retrofitted over a period of several months about 1-2 years back. The original threads should pop up on search but I think it is difficult to find as the key discriminator in the thread title is 'M' and I don't think the search function looks for single letters or letter/number combinations.
As I recall the guy did a bit of "home electronics", possibly made his own circuit board. ???
I have never got round to having it done myself but I do miss the cruise control I had in the 3.0l. However I have visited the garage a couple of times and although it has a great reputation I didn't like the main guy's (owner) attitude when I asked about bleeding the brakes on changing pads, (this was around the time some other owners had had calliper seizures after pad changes). He was dismissive and didn't give off the vibe that he really gave a s**t about my car. Having said that the actual mechanic who worked on it was superb, very gentle and controlled with his tools and the work has been faultless so perhaps I judged a bit hard. Eiher way the experience was enough to stop me from going back and put me off the idea of allowing them to dismantle part of dash and fit some home made electronics that BMW say can't be done.
I'm sure some other members who had it done will be along soon with their tuppence. . .