Butler, nicknamed 'Big Zo' thanks to his 1.91m by 113kg frame, brings a 30-2-1 record to the party.
Ritani-Coe (3-2-1) is the only man to have gone the distance with Kiwi heavyweight Joseph Parker when Parker took a points decision from their six-round bout in California last year.
Banks, who last fought in June and has a record of 29-2-1 with 19 knockout victories, has also dabbled at cruiserweight previously.
He told the Herald he may make the trip to New Zealand and would likely decide whether to pursue the final Super 8 spot in the next day or two.
It's understood Banks was informed about the tournament during the lead-up to Klitschko's title defence last week and took an interest.
"It's not an ideal situation," he said of the format. "But if all the ducks are in a row and everything goes right, it's possible."
While it hardly sounds like he is desperate to be involved, the tournament would be a way for Banks, who was previously ranked second in the world by the WBO, to keep active given Klitschko will now take some time off following his victory over Leapai.
Super 8 spokesman Mark Keddell said a formal approach hadn't been received but they would be keen to have another fighter of his calibre on the bill to join the likes of former heavyweight world champion Hasim Rahman.
"We have one spot to go and would definitely be interested in talking to him," Keddell said of Banks.
Banks only fought once in 2013 but said he would be ready in June if required.
"I'm ready to start sparring. I'm in pretty good shape right now. I just came from a nine-week camp with Wladimir."
Kali Meehan, Samuel Peter, Michael Sprott and Martin Rogan have all been confirmed, alongside Rahman, Ritani-Coe and Butler for the June 4 tournament, which has a purse of $500,000.
Fighters will compete in rapid-fire bouts of three three-minute rounds in a bid to find a winner.
Sky Arena also announced their undercard fights yesterday with Ant Nanson set to face Thomas Peato in heavyweight action, while undefeated super middleweight Joe Blackbourn will meet Dimitri Simoukov.
Cruiserweights Sam Rapira and Taito Ratuere will square off and heavyweight prospect Hemi Ahoi will battle William Quarrie.
