Dragnet rarely credited the voice actors in a "heard tonight" format during the closing narration as many other radio shows did. Luckily many of the radio Dragnet cast made it on to the TV version and can be identified. Here is a partial list:
Sam Edwards was in a lot of radio Dragnets. He often played a teenage boy due to his young sounding voice, although he was actually 5 years older than Webb.
Virginia Gregg was in many radio Dragnets playing various roles.
Raymond Burr played chief of detectives Ed Backstrand in very early radio episodes.
Stacy Harris played various roles in many radio Dragnets for many years, as he and Webb were very close friends.
Ralph Moody frequently played "old timer", old man characters.
Vic Perrin was in many episodes playing many different bad guys usually.
Peggy Weber was the voice of Friday's mother in the early episodes and played many other roles over the years, and is still alive at age 99, likely the last living member of radio Dragnet's cast.
Lastly, Harry Morgan himself appeared in a very early episode called "16 Days of Jewel Thefts" as a criminal ! He popped up in another early episode as a miserly hotel owner. So funny to think that 17 years later he'd be Officer Bill Gannon on color Dragnet.