There are some shops doing this, here is one: http://www.pro-rv.com.sg/
I don't know how much - I guess it'll be case to case basis, which
sunroof model you choose, etc. I don't know how long either, but I
suspect that it'll be a few days to a week (they have to cut,
install, repaint, etc). Best if you checked with the shops
itself.
I've had a car with sunroof before (an old Honda CRX) and honestly,
its not worth it. As the driver, the sunroof starts behind your
field of vision, so you won't actually notice it. Its more for the
back passengers to enjoy.
Its also very noisy and hot if you open it in Singapore. In the
end, the only time I opened my sunroof was when I wanted to
smoke....I could blow the smoke upwards through the
sunroof....instead of sideways out the window. 
You might want to know that there have been some complaints too,
that aftermarket sunroofs sometimes leak.
