Wagner split the webbing in his bowling hand while fielding and missed the last two matches.
Cumming badly sprained his ring finger, also while fielding, and had to sit out Otago's two-wicket win over Auckland in Oamaru on Sunday.
It is not all good news for the Volts, however, with experienced pace bowler James McMillan ruled out.
He has a sore hip and has been given the week off to rest.
Sean Eathorne, who replaced Cumming on Sunday, drops out of the 12, with Wagner taking over McMillan's spot.
"Getting Craig back is obviously pretty helpful for us," Otago coach Mike Hesson said.
"He missed the last game and I thought Sean Eathorne did a super job and has put pressure on some of the other batsmen, as well."
Initially, Cumming's injury appeared more serious.
He was told it could take four to six weeks to come right, but has made a speedy recovery.
"He has had a couple of good sessions with us and has got through. When he doesn't hit the ball right, it hurts a little bit, but it is only pain. He just has to get through that. The key thing is it is not going to cause any further injury."
Otago had a dreadful twenty/20 campaign but has bounced back and, after five rounds of the one-day tournament, is in pole position.
"To be fair, we've played good cricket all year, bar about two weeks in December. So it's nice for the boys to get the confidence back."
One more win would be enough to secure a playoff spot but Otago is aiming for a home venue, and Hesson believes two wins from the remaining three round-robin games will get the job done.
There is still plenty of work to do before the Volts can contemplate a home playoff.
But if they succeed, the game would be played at the Queenstown Events Centre.
Northern coach Grant Bradburn has named an unchanged squad, with young batsman Kane Williamson to play today before heading to Christchurch to play for the Black Caps on Saturday.
The Knights won their first two one-day games but have since had a loss and two no-results, so will be keen for a result today.
