They join "Torchwood" regulars John Barrowman as Capt. Jack Harkness and Eve Myles as Gwen Cooper in the series, which is to debut July 8. It is currently in production in Los Angeles.
The cast will also include Alexa Havins and Arlene Tur.
The latest series in the "Torchwood" science-fiction franchise begins with a day when nobody dies.
"All across the world, nobody dies," Starz said in a synopsis Wednesday. "And then the next day, and the next, and the next, people keep aging, they get hurt and sick, but they never die. The result: a population boom, overnight. With all the extra people, resources are finite. It's said that in four months' time, the human race will cease to be viable. But this can't be a natural event -- someone's got to be behind it. ... The answers lie within an old, secret British institute."
Set in Cardiff, Wales, "Torchwood" ran for two seasons, then was followed by the miniseries "Torchwood: Children of Earth." The previous incarnations aired on the BBC and BBC America.
"Miracle Day" will be the first installment to take place in the United States and debut on Starz.