Looks like Morgan also wrote the follow-up episode "Jose Chung's Doomsday Defense" for Millennium. I think CNR played the first character to appear on both The X-Files and Millennium.
Darin Morgan is the brother of Glen Morgan who was a prolific writer for The X-Files and one of the show runners for Millennium season 2.
I've been listening to The X-Files Files
podcasts after I watch the episodes and it had interviews with Morgan after Clyde Bruckman's Final Repose and Jose Chung's From Outer Space. These interviews are easily my favorites of the podcasts. Morgan has an odd sense of humor but still seems very down to earth.
He also played the role of the Flukeman in the classic episode The Host and he was the shape shifter in Small Potatoes.
I remember giving Millenium a try when it first aired, but it never captured me like The X-Files. I should watch it again.