---========================================================
-- table definitions

-----------------------------------------------------------
-- destination table for new data to be inserted
DECLARE @Destination TABLE (
   Id INT NOT NULL IDENTITY(1,1)
      PRIMARY KEY CLUSTERED
   ,SomeInt INT
);

-----------------------------------------------------------
-- table containing all rows to be newly inserted into
-- destination table
-- IMPORTANT
-- the table containing the new data to be inserted requires
-- any kind of uniqueness (e.g. a surrogate id)
DECLARE @NewData TABLE (
   SurrogateId INT NOT NULL
      PRIMARY KEY CLUSTERED
   ,SomeInt INT
);

---========================================================
-- create some sample data

-----------------------------------------------------------
-- insert existing data into destination table
INSERT INTO @Destination
             SELECT -777
   UNION ALL SELECT -888;

-----------------------------------------------------------
-- new rows to be inserted into destination table
INSERT INTO @NewData
             SELECT -1, 11
   UNION ALL SELECT -2, 22;


---========================================================
-- bulk insert rows into destination table and catch new identities

-----------------------------------------------------------
-- variables to catch count of inserted rows and scope-identity
DECLARE 
   @LastIdentity INT
   ,@RowCount INT;


-----------------------------------------------------------
-- bulk insert new rows into destination table
-- IMPORTANT
-- to get the following operations work ensure to use an
-- ORDER BY in SELECT. To avoid problems due to paralellism
-- use MAXDOP 1 option what should be okay for an INSERT
INSERT INTO @Destination
   SELECT SomeInt
   FROM @NewData
   ORDER BY SurrogateId
   OPTION (MAXDOP 1);


-----------------------------------------------------------
-- catch the last created scope-identity and the count of
-- rows inserted
SELECT
      @LastIdentity = SCOPE_IDENTITY()
      ,@RowCount = COUNT(*)
   FROM @NewData;


-----------------------------------------------------------
-- use a CTE that creates a row number ordered by the 
-- surrogate id of newly inserted rows
WITH cte AS (
   SELECT
      SurrogateId
      ,ROW_NUMBER() OVER (ORDER BY SurrogateId) RowNum
   FROM @NewData
)
SELECT
      *
      -- calculate destination table IDENTITY values
      ,@LastIdentity - @RowCount + RowNum
   FROM cte