Convert SQL server stored procedures to postgreSQL
Unfortunately you'll probably have to handle the stored procedures by hand. If you're lucky, they're simple procedures following standard SQL syntax, and you'll be able to just copy the existing queries over to PostgreSQL, otherwise you'll have to handle any fallout on a case by case basis.
Side note: this is one benefit of some database agnostic ORMs like Dapper, if your database is mainly consumed by application code. By using the ORM framework's native calls, you decouple your database logic from the actual database system, but there's pros and cons.