Federline, 33, already has two sons with his ex-wife Spears, and a son and daughter with his ex-fiancee Jackson.
People.com said Federline's longtime girlfriend, former volleyball player Victoria Prince, is pregnant with their first child.
"Honestly, I wish them the best, without question," Jackson told People.com Tuesday.
"It's just not part of my life," she said. "I've got my own family to worry about, I can't really be worried about what's going on with anybody else. Good for them, and, of course, I wish them well."