Joining them on the reality show will be ex-Pussycat Doll Melody Thornton, Toploader's Joe Washbourn and Erasure's Andy Bell.
Completing the lineup are Steps star Claire Richards and US soul singer Jocelyn Brown.
The ITV series pits pop singers against each other in a competition to learn opera singing. Darius Campbell triumphed in the final of the first series last year.
Myleene Klass will return to host the show, but the judging lineup - which last year included Laurence Llewelyn-Bowen, Meat Loaf, Katherine Jenkins and Rolando Villazon - has not been revealed yet.
McElderry was reportedly dropped from Simon Cowell's Syco record label last month.
Popstar to Operastar returns to ITV1 in the summer.