As far as I know, Isofix cannot be retrofitted to a car - it can only be installed at the factory. I don't think you'd find a dealer willing to weld the brackets in. You would also need a new seat base with the Isofix covers. Imagine the potentially horrific consequences if a weld fails and the subsequent public liability claim.
It's scaremongering to say that a non-approved seat will invalidate your insurance. Child seats cannot be tested in every make and model of car in the world. Generic tests are made to ensure basic safety. Any BMW dealer who spouts this misinformation is lying.