Oscar, Emmy and Tony Award-winning actor Jeremy Irons and series alum Diane Neal will also return to make guest appearances in upcoming episodes, the network said.
Stamos will appear in the episode titled, "Bang," as an adoption attorney with a secret passion.
Singer, in a rare television appearance, will join Stamos, playing a new mother struggling with her increased responsibilities.
Howard will guest star in the episode, "Reparations," as part of a crossover episode between his show "Law & Order: Los Angeles" and "Law & Order: Special Victims Unit."
Irons, who appeared in an episode this season, will reprise his role as Capt. Jackson, a psychologist who worked with the SVU team. The episode, titled "Totem," will "showcase the first case in series history to feature a solo female perpetrator responsible for sodomizing and murdering her victim," NBC said in a news release.