To add SA user to NAV database running on Microsoft SQL Server with SUPER role you will need to execute following statements. The scripts below presumes you have access to database login SA. If not - replace SA with another known SQL Server login. 

USE [your-NAV-database-name-here]
GO

INSERT INTO [dbo].[User]
 ([User ID]
 ,[Password]
 ,[Name]
 ,[Expiration Date])
 VALUES
 ('SA',
 '',
 '',
 '1753-01-01') 

INSERT INTO [dbo].[Member Of]
 ([User ID]
 ,[Role ID]
 ,[Company])
 VALUES
 ('SA',
 'SUPER',
 '')
GO