There was some discussion on the NetBSD list today as to how you might do this properly, ie with OS support not some hacks [1]. Although it is not clear it is worth bothering, as you just use screen first...
* press ctrl-z to put the program in the background
* start screen
* run fg within screen
(1) ^Z sends SIGSTOP; to continue you can signal SIGCONT. But then what?!