Former South Africa captain Du Preez has replaced compatriot Dane van Niekerk in the Southern Vipers squad.
Van Niekerk, who took over the Proteas captaincy reins from Du Preez, was forced to withdraw from the tournament due to an injury sustained at the World Cup.
Du Preez had a solid World Cup with bat, finishing as her side's third leading run-scorer with 229 runs in seven games at an average of 57.25.
Bob Parks, Southern Vipers general manager, said: "I consider ourselves fortunate in that we have found such a talented and experienced player to replace Dane van Niekerk.
"She will certainly add value to our batting unit and comes with a high reputation as an excellent fielder."
Another of Proteas star at the World Cup is opening batter Lee, who has been signed by the Surrey Stars to replace India's Harmanpreet Kaur.
Lee, 25, hit 269 runs for South Africa in the World Cup, at an average of 44.83.
"After enjoying my time at the recent Women's World Cup, it's fantastic to be involved with the Super League - and especially the Surrey Stars," said Lee.
"With seven full internationals - as well some of the most exciting young players in the country - it will be a great environment and I look forward to playing my part and hopefully helping the team reach finals day in Hove on Friday, September 1."
Lee and her fellow South African Marizanne Kapp - who was named alongside Beaumont, Hartley and Sciver in the team of the tournament - are also due to report for duty later this week.