You can define the update statement in the target. There is that properties. Still you have to make informatica to perform an update, not insert. To do that you need to use the update strategy. I think you don't need in this solution to make any PK on that table, because you will use your own update statement, but please verify this.
To set the fields and make proper where condition for update you need to use :TU alias in the code. TU -> means the update strategy before the target.
update t_table set field1 = :TU.f1 where key_field = :TU.f5