Justin will play Neil, a love interest for Gilbert's original sitcom character Darlene, according to Deadline.com.
"The Big Bang Theory"'s Johnny Galecki is set to return as Darlene's ex-husband David, with Juliette Lewis tapped to portray his girlfriend Blue. Ironically, the two former child stars once played brother and sister in 1989 comedy "National Lampoon's Christmas Vacation".
"The Conners" was picked up for a 10-episode order by ABC network bosses in June, weeks after they cancelled a planned second season for the revived "Roseanne" show following leading lady Roseanne Barr's racially insensitive Twitter controversy.
Producers, including Gilbert, rushed to revamp the show as a spin-off in a bid to save the series, which became a surprise hit, and now "The Conners" will premiere in the U.S. on 16 October.
Barr is not making an appearance on the new show, which also features John Goodman and Laurie Metcalf, and according to the veteran comedienne, her character has been killed off after suffering a prescription drug overdose.