I know some guys on E46 Fanatics have done an SMG paddle retrofit from the M3. It's about a $1,500 conversion as the paddles, steering wheel and airbag are needed in order to do the retrofit. That having been said, the paddles in the M3 are quite a bit different than the ones found in the 335i and Z4.
I don't know where your friend had hers done but AFAIK this was not an option from the factory. Fleet might be able to offer a little more insight on this one.
2006 BMW 325I
Sparkling Graphite | Black
Sport Package | Premium Package | Xenons