I have a stored procedure 'sp1' on Sql Server.
I have two issues;
 Once I create data connection and pull data using sp1, I am unable to add more tables(master tables) with sp1. When I am selecting another table, it replaces the existing stored procedure.
 How can create custom query for stored procedure. My database name is 'test'.
Tableau can use something called Custom SQL. You can add your needed SQL script to extract the data you need: Connect to a Custom SQL Query
As far as what that script should be, that's going to be difficult without seeing some sample data/schemas. Stackoverflow is a good place for those types of questions if it is not answered here.
Hope this helps!