It is also possible in case the part has an international approval. See the very end of MKBT 11 what information for example is needed in case of type approved exhaust.
In some cases you can not get those papers from the manufacturer but from the official importer or an other importer for Switzerland only. In such a case the importer my charge you a fee. A rough estimate is CHF 300 per paper, give or take.
In the worst case neither manufacturer nor importer can give you the right paper. In such a case the third option might be to get an approval from a so called Anerkannte Prüfstelle. There are only few of those: For general car parts it is the DTC and FAKT. They charge you an arm, a leg, plus your first born for the teste and paper. This might not be possible if drawings and specifications are not available.
Tom
I got quoted nearly that to sort out the paperwork of an unmodified car I wanted to bring in...
My understanding of your situation is you will need to convert the car to stock, get the original paperwork to import it, and then put your mods back on and then get those type-approved (if possible - some things can't be done).