You can import your existing NoSQL Databases to RavenDB.
You can import all collections, or select specific collections to import.
In this page:
To migrate data from your existing NoSQL database, you need to download an external tool called Raven.Migrator.
The application is available at ravendb.net/downloads, in the tools package.
Import from MongoDB
Migrate all collections
Use transform script
Import from CosmosDB
Azure Endpoint URL