@k_b, thanks a lot! I now have the first 12 Motets ready in the shape, they were printed in “Sämtliche Werke I” (1904). So far I didn’t add the text, because I don’t want to sing the pieces.
The second part of the original publication are 12 duets without text (“Sequuntur Cantiones sine textu”). The link on IMSLP lead’s to the “Bicina”, you posted, but these are from a different publication. Thererfore the link is somehow wrong. I’ve got a copy of “Sämtliche Werke”, so I will type them in myself and I will also look through the Bicinia. At the end I will have a lot of material to choose from …
Meanwhile I also found most of the pieces on CPDL as Sibelius files. I was so focused on the idea, to find a way to transfer the Lilypond files to Dorico, that I forgot to look any further. 
I really love this forum, because there are so many helpful people around here! Thank’s again to everyone, who spent some time to help me.
If anyone is interessted in the result, I could post the file here. ( … when it’s ready …)
Hi Andro, I may be wrong but what came me across of your comment is that, IMSPL files csn be easily converted to Dorico. It this true? And if so, how? Are they not simple PDF files?
I think the point was that this particular piece is not very long and could be input directly into Dorico as a new project without too much effort – not that it could be converted from whatever is on IMSLP.
Oh, I see! So no trick, just the slavery of putting the heavy notes into the coach:))))
Yes, just that it is not long. There is no conversion tool available.
Gergely,
if you don’t like to re-input the music into Dorico, you can try to use a PDF2Music.xml application to convert the PDF files to music.xml.
This does sometimes work quite well and can save you a bit of time - but you still have to go through the whole piece and check for errors.
They can make up from 5% to almost 100%.
Hi k_b, can you recommend me some? I found Capella Scan what I can download for free, but they are upgrades for a fee, and usually those free stuffs are working only if I upgrade it. I don’t mind paying, but only if it is really worth. Is there any free? Or you have good experiences with Capella?
There has been two very good articles by Scoring Notes about optical recognition software in music, I’d recommend reading them 
There are some alternatives. and they are well described and linked to in a couple of forum threads.
I bought Photoscore Ultimate (there doesn’t seem to go much on in development..)
and also PDF2Music it works quite well but excludes all scanned documents (as there are lots of these around)
PlayScore 2 is my favourite in recent months. It is in constant development, it runs on iOS or iPadOS or Android. There seems to be a Windows version available.
For the first two mentioned, you pay only once - no subscription. For PlayScore Professional, I am happily paying a yearly fee.