In merge statement matching and update on the same column is not possible.
This behaviour is same irrespective OWB or SQL.
If your requirement is just the sql statement that you have provided you can implement it in the following way owb.
Craete mapping with a table (same for source and target).
Use the filter operator to imeplement the condition "columna = to_date('19/05/2009','DD/MM/YYYY')".
Set the target type as update and run the mapping.