IF NOT EXISTS (SELECT * FROM SYSCOLUMNS INNER JOIN SYSOBJECTS ON SYSOBJECTS.id = SYSCOLUMNS.id 
               WHERE SYSCOLUMNS.NAME = 'DOSYATESLIMNO' AND SYSOBJECTS.name = 'HALLIANZDOKUMAN')

ALTER TABLE dbo.HALLIANZDOKUMAN ADD DOSYATESLIMNO NVARCHAR(50)  NULL
go