hi Damien,
Currently you can join the source with the target table as you did, or you can setup the target table twice: one for Insertions and the other for Updates.
I'm going to transfer this need to our r&d team, I feel that we can add something in a future version to make this easier...
Edit : you can try to use the incremental flag in your expression, see the attached screenshot. Note that the method and syntax can vary depending on the Rdbms technology and on the selected Template.
Thanks
Thomas