A sunroof is not usually considered to adversely affect insurance and they are easy to get fitted as an after-market add-on. Just be careful to choose a reputable fitter with a solid guarantee. If your BMW is relatively new, then try going to an authorised agent and seeing who they get to do theirs.

SHARK! 

I have just bought a 2nd hand 3 series BMW.  Would ideally hsave liked one with a sunshine roof, but couldnt find what I wanted.

Is it possible to get one retro-fittd.  And if so does it mess up your insurance (Ive noticed that for quotes they tend to ask "has your car been modified")