If you don't use an IDENTITY column you can import the date into a temp table and use the following statement to add an incremental id:

IF (OBJECT_ID('tempdb..#test') IS NOT NULL)

DROP TABLE #test

CREATE TABLE #test (id INT, txt VARCHAR(100))

-- Some test data

INSERT INTO #test

SELECT TOP(100) NULL, name

FROM master.sys.columns

-- Create an incremental id

DECLARE @id INT

SET @id = 100

UPDATE #test SET @id = id = @id + 1

-- Result

SELECT * FROM #test

Now you can INSERT the data into the destination table.

Greets

Flo