Hey, thanks for reaching out!
We have the Import / Export feature available. With this you could create a JSON from the documents you have in Netlify and then use the correct structure to import all documents to Prismic. Look a tthe complete reference:
Unfortunately we haven't explored migrating a project coming from NetlifyCMS, so for that I wouldn't have other best practices to share with you for this use case. Hopefully someone from the community will share their experience if they have done a similar thing in the past.
Let me know if Import / Export would be a good solution for you, and if you try it out let me know how it goes.