You can put manually custom campaigns in this folder in case you're wondering if it's possible to do it manually :
"...\Documents\Parkitect\Saves\Campaigns"

I tried with the ones you can find here https://mod.io/g/parkitect/m/realms-of-adventure and here https://mod.io/g/parkitect/m/rct-campaign and it seems to be working well :)

Edit : and if you want to learn how to create your custom campaigns :)
https://www.youtube.com/watch?v=fyBrLSmkHAc