Unattended call transfer

How to make a call and transfer before one of the extensions answers.

It sounds like what you are trying to do is a third party call set up, not a transfer. I doubt that FreePBX supports that within the GUI. The easiest way of doing it with Asterisk is with Originate, although the process isn’t symmetric. For a symmetric set up, I think you would have to put both into a conference bridge, with two originates.

How could I merge the conference with two sources or destinations?

Originate to both with the conference as the B side. I doubt that you can do this without custom dial plan.