If you are using Data flow task and just using Oracle Source and SQL destination, the insert will fail becoz the task is trying to insert new records. As you said you already have data in your SQL table you just need to update the other columns with values from Oracle. There are many ways to accomplish this: You can use whichever you like based on size of your table (for performance)

1. Add slowly chaning dimention task to your data flow and update your SQL table based on your primary key.

2. you can loop through the recods from oracle using foreach loop and update the sql table records

3. you can bring all data from Oracle into staging and then write simple sql update statement to update all the columns in your sql table.

HTH

~Mukti