Use of third party ad 2 drummer kit in synfire

Depends how it works. 

If it just provides sounds you can use it like any dumkit sound. 

If it plays its own patterns synchronized with the timeline, it should also work but it will not recompose its patterns to match your current narrative and container structure in Synfire.

If you can export its patterns to MIDI files (drag and drop?), you can batch-import all of them into a library of course.