It's mostly impossible to transfer miles between frequent flyer programs. But you can use Voyager miles to redeem flights on any *A airline - like United, for example. Best is to find the availability for a route and date that suits you (using a decent redemption web site like United's MileagePlus web site or Avianca's LifeMiles web site). Then call Voyager to issue the redemption tickets. You can only book return journeys on *A partners; you can book one-way journeys on SA.

Remember that Voyager miles expire after 3 years unless you maintain a minimum earning level every year. So make sure you don't let that stash disappear on you.