can cruise control be installed outside of dealership without voiding the warranty

not sure but I seriosly doubt it....

No.

Any external device but on the car that wasn't part of the orginal package will void the warranty if done by any one except a certified bmw dealer.

if you are afraid of the warranty, you could ask bmw to do it ?

Yes it can, go to Z roadster.net and search the threads, It just plugs in on the steering column wiring up to a certain manufacturing date, after that you need a harness too, can't remember the date tho, my Feb 05 one did't have the wiring there.

Shame - that's the one useful extra that's missing from my 04 3.0i.