I try to create a book of songs with a professionnal printer.
He ask me to add bleed (“bords perdus” in french).
For some image near the limit of the page, he want the image continue in the bleed, to be sure there isn’t blank space after cut.
This is not possible with Dorico currently.
I would suggest printing to PDFs in Dorico as usual, and then blowing the PDFs through another printing process where you can add a few mm’s for the additional border and add the cutting marks.
Do you actually have images right to the page edge in Dorico? Normally, the page margins prevent anything going to the edge.
Dorico cannot create a bleed area. As @Estigy suggests, you would have to put the Dorico page into Affinity Publisher, InDesign, or other DTP app – and then you would have to make sure that the image ran over the page edge, into the bleed area in that app.
Your Printer will cut the page to an accuracy of ±1.5mm. You need to make sure that wherever the blade falls, it looks ok!