Entity Framework allows you to extend the DbContext class to add your own entities, extend existing entities, and customize them at runtime. All of these in a clean and simple manner, if you are willing to do so. It doesn’t even matter if those entities don’t have a table representation. You can do that by leveraging C# partial classes and method.
How to create migrations in EF Core with SQL Server provider? How to handle migrations with EF Core? Migrations in a different library than your main project?