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

Well, if it's an existing database, you could just use Microsoft SQL Server Management Studio to access the stored procedures. They can be found under 'Your database' > 'Programmability' > 'Stored Procedures' in the Object Explorer. Though I'm pretty sure that using both stored procedures AND entity framework is not common practice.

To answer your question, right-click the Stored Procedures filter in the Object Explorer mentioned above and click the Stored Procedure option to add a stored procedure to your database.

If you don't want to use MSSMS, you can also add stored procedures in C# using the migration functionality that entity framework provides, explained in this question.