This content is for an older version of Synfire. The current version is more streamlined and advanced, but most of the functionality shown here still exist.
This video demonstrates from scratch the steps to relocate a VSTi (here: Halion Sonic) from a Synfire Engine to a drone hosted at the DAW (here: Cubase).
As you can see in the video it is necessary to set the sound categories and the sound names before you can relocate the VSTi since otherwise the 'relocate' button is greyed out.
Note: Due to an issue at my sound setup the screen capture program did only record the sound from the Synfire Engine but not the sound from the drone at Cubase. That's only an issue at the video. In reality the sound was there.