Add Data from Columns Together
You answered your own question, by importing it into a temporary or staging table. 

CREATE TABLE #TempTable(
    job_id   INT NOT NULL
    ,part_id  INT NOT NULL 
    ,quantity INT NOT NULL)

BULK INSERT #TempTable
FROM 'C:\example.csv'
WITH
(FIELDTERMINATOR = ',',
ROWTERMINATOR = '\n') --any other options like FIRSTROW, etc


Then, just use the SUM() aggregate when inserting into your actual table

INSERT INTO MyTable
SELECT 
   job_id   
   ,part_id  
   ,quantity = SUM(quantity) 
FROM #TempTable
GROUP BY
   job_id   
   ,part_id