Yes you can.
It is possible to add custom page sizes to Dorico. I’ve added JIS B4 to this doricolib file:
Additional Page Sizes.doricolib.zip (1.3 KB)
You need to add this to a folder named “DefaultLibraryAdditions” in the folder where your Dorico user preferences are stored, e.g. on a Mac:
[User]/Library/Application Support/Steinberg/Dorico 5/
and then restart Dorico.