We've jut installed one in a wall,it was a real faff and dh is REALLY good at DIY.
I read up on getting them fitted in windows- you can but it's expensive and although some companies cut holes in existing double glazing it means the seal is then weak and prone to cracking. There are companies that do this so.
Can't see how it can be fitted to a upvc door but might be wrong.
I'm sure a handy man would be used to doing this sort of thing though.