This is possible. They've told me to fit it with just bolts inside initially. Once it's in I can retrofit something like this. Might actually be the best way as I'm less likely to screw something up due to my inexperience if I can see the whole thing in situ first. I'm sure when you've done this a few years you can just look at the door opening and have the whole thing planned in your head ... that's sure not where I am!