How to add stored procedures to existing database model (in Code first Approach)?

You may simply access the stored procedures using Microsoft SQL Server Management Studio if the database is already in place. In the Object Explorer, you may locate them under "Your database" > "Programmability" > "Stored Procedures." Although I'm quite sure it's not typical practise to use both stored procedures AND entity framework.

To add a stored procedure to your database, under the Object Explorer's Stored Procedures filter, right-click it and choose Stored Procedure from the menu that appears.

If you choose not to utilise MSSMS, you may alternatively implement stored procedures in C# by using the entity framework's migration feature, as detailed in this inquiry..