Kaber, a left-arm wrist-spinner, has been part of MI Cape Town squads for the last two seasons, including the the title-winning campaign in season 3.
The 33-year-old has good record in the shortest format having taken 61 wickets in 57 matches at an average of just and 20 and an economy-rate of just above seven to the over.
Kaber join a strong contingent of spinners at MI Cape Town with Rashid Khan, George Linde and Dane Piedt already in the squad.
Proteas spinner Shamsi was bought for R500 000 by MI Cape Town in the auction that was held in September but has pulled out of the tournament.
Shamsi had previously represented Joburg Super Kings last season and Paarl Royals for the first two seasons of SA20.
The fourth season of SA20 kicks off on 26 December when defending champions MI Cape Town take on Durban's Super Giants at Newlands.
Sunrisers Eastern Cape underlined their status as SA20's dominant force with a nerve-shredding six-wicket victory over Pretoria Capitals at Newlands, clinching a third title in four seasons.
Sunrisers Eastern Cape sailed into yet another SA20 final after beating the Paarl Royals by seven wickets in a low-scoring encounter at the Wanderers on Friday.
Lhuan-dre Pretorius found his form at the perfect moment as Paarl Royals ended Joburg Super Kings' SA20 campaign with a commanding 36-run win at Centurion on Thursday night.
A commanding display from Pretoria Capitals' young stars Dewald Brevis and Bryce Parsons propelled the team into their second SA20 final, as they overcame Sunrisers Eastern Cape by seven wickets at Kingsmead, Durban, on Wednesday night.
