Try modifying Content/Data/SecretNotes.json, you should just be able to add another entry to the dictionary there for secret notes you get via the hoe.. and other methods?
Are there other methods to finding randomly generated notes?
I dunno.
For secret notes that are behind bushes and stuff, I think those are hard coded, so you would prob have to do some c# shennanigans.
(If they arent, please correct me bc that would be so much easier ;u;)