The 35-year-old star has two daughters - Phoenix Chi, 11, whose dad is her ex-husband Jimmy Gulzar, and Angel, three, whose dad is actor Eddie Murphy.
She told Britain's Buzz magazine: 'Stephen's been my friend for eight years, so him joining my family when we got married three years ago felt like a natural progression.
'My youngest daughter Angel, three, calls him Dad, even though he's not her biological father. Our three girls - Angel, my 11-year-old Phoenix and Stephen's daughter Giselle, who's six - get on really well.
'But we definitely want to have a child together in the next year or so.'