I posted a walkthough of how I did it in this thread. Keep in mind Dorico (as of D5.1) only looks for the pageset.fullscore and pageset.part sets, so you can’t simply add additional sets. If you name your exported sets those, you’ve basically overwritten the factory sets with your own sets, so the factory page templates will not appear, unless of course you’ve exported them with your own sets that you renamed. (It’s easy to revert to factory by removing your doricolib files though.) You can only have one default set of templates for score layouts and one for parts, but it’s fairly easy to edit and customize these with a doricolib file.
I’m spending the next couple of days doing part layout for a ballet, and it sure is great to have all my default V.S. and Page Turn page templates ready to go with just a click!