Olá, Flavio

Felizmente você pode fazer a "engenharia reversa" de um banco de dados preexistente, utilizando o comando Scaffold-DbContext do Package Manager Console, conforme o documento da microsoft:

Reverse engineer your model

Now it's time to create the EF model based on your existing database.

• Tools –> NuGet Package Manager –> Package Manager Console
• Run the following command to create a model from the existing database. If you receive an error stating The term 'Scaffold-DbContext' is not recognized as the name of a cmdlet, then close and reopen Visual Studio.

` Scaffold-DbContext "Server=(localdb)\mssqllocaldb;Database=Blogging;Trusted_Connection=True;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models

https://docs.microsoft.com/en-us/ef/core/get-started/aspnetcore/existing-db