The Sixers, meanwhile, released wicketkeeper Peter Nevill to make room for Smith in the team.
The 9th edition of Big Bash League starts from December 17.
These Australian stars have been scoring heavily in the BBL so they deserve their due credit in the form of an IPL contract in the upcoming auctions.
Alex Hales has signed a one-year deal and will join Chris Morris as the second overseas player with the side.
Speaking ahead of his new journey, the legendary batsman said he is excited to play in the Big Bash League.
The new rule applies to the tied finals of BBL 2019-20.
If the Stars manage to sign the South African star, it would prove to be a huge marketing and business boost for the franchise.
Faheem Ashraf and Mohammad Hasnain also approached by Big Bash sides
It was earlier reported that De Villiers was close to inking a record deal with one of the teams.
The rule of only 2 foreign players in the playing XI will be intact despite roping in 6 overseas players which was 4 previously.