you cannot update with sql*loader.
you can load the data into a similiar staging table using sql*loader.
and apply your update logic from the staging table to the target table.
drop the staging table.

you CANNOT UPDATE using sql*loader.
UTL_FILE is meant for other purposes.
to implement a logic using UTL_FILE would be tedious.
in your case,i tried this. it worked fine.

1. TABLE1 is target table.
2. TABLE2 is a staging table.
3. load into staging table using sql*loader.
4. Before loading,
   create an after insert trigger on staging table.
   in the trigger, construct your logic for updating
   the target table using the records inserted in staging table.