there was a roof Available from Webasto for the Voyager, with Paperwork (for TÜV) and everything!
Its the same as used in the VW T4. Its name depends on the Year is is built, either "Top Slider Medium" or "Hollandia 300 Classic".
See the Wiki for further information:
Does everything need to be TÜV approved and such? Like if you bolt in a new seat or weld on a new top?
Is it expensive/difficult/not worth it?
Yes, a sunroof and especially seats have to be TÜV approved.
However if you speak to an Engineer and TÜV and talk about what’s what you wanna do a Sunroof can be put in the Papers via Paragraph 21 STVO