Hi,

Have to get the health warning out the way and say I haven't done this myself, however, in principle it should be the same process as fitting side windows. 

As it's the roof I'd say the issue of making sure it is sealed and weather tight becomes super important.  It would definitely really help with adding some great natural light to your conversion though.  I'd make sure I had a window / skylight which was designed for your van make and that specific purpose as a starter.  As long as you cut the shape accurately and get the seal tight with a good sealant this should be fine.  Make sure you leave it to cure properly and it's probably something you'ld need to redo eventually as if the van is out in extremes of weather, like any other rubber / sealant it will eventually perish (but we're talking years if you get it right first time).  If you do it, it would be great to see some photos.  Good luck!