I'm no world expert, but I'll throw my 2 Szloty in anyway
K100s ( 1100s etc) do not have a full cradle frame, so a sub frame needs to be made up to attach the sidecar chassis to. That requires good design and fabrication skills. A sidecar manufacturer and fitter could do that, but it won't be cheap. I suppose you could say there are 2 main possibilities for fitting a chair.
#1. go down the route of having a professional make the brand new sidecar, fit it to the BMW using a sub-frame etc. This will involve the cost of the new chair as well as the fabrication of the sub-frame etc. Going this way would also give you the option of having a leading link front end made up and fitted as well, and [possibly the fitting of a car rim to the rear of the bike for better contact with the road and( much) longer tyre wear. This route would not be a cheap exercise, and would cost way more than the cost of the K bike in the first place. That might be acceptable though, depending on how badly you want the sidecar etc.
Melbourne sidecars might be the closest option to Tassie?
http://www.sidecars.com.au/html/s01_home/home.asp#2. Find yourself a second hand DJP or HRD or whatever you can find and do it that way. These old Aussie sidecars are very robust, reliable and perform just as well as anything you could buy brand new. ( just my opinion of course) Sadly though, these are becoming harder to find these days and are priced accordingly. You'll still have to find someone who can fabricate the subframe etc, unless you have those skills yourself. If you do, let us know!
There are plenty of posts on here in regards to fitting a chair to a K bike. Do a search and see what you come up with.
Cheers
Sean.