Well, I finally found the complete solution to my seat back (my back actually) problem from other forums. I followed hints and links in the German
http://www.e90-forum.de forum:
E92 lumbar support retrofit
E90 version (English)
E90 version (German part 1)and
Part 2
E46 version
Now I'll have a go at converting my sports seats.
Cheers,
Joe