To add custom instruments, you need to modify some XML files under
/Applications/Dorico 2.app/Contents/Resources. Only do this if you're comfortable doing something like that!
I have included the complete file contents here so you can just replace the existing files on disk. You will overwrite the following files in the Resources folder:
instrumentnames_en.xml(or add entries for your specific locale - see bottom of README)
This configuration will create a new "Parts" instrument group at the top of the list. It provides three instruments: