The 22-year-old is expected to replace outside-half Frederic Michalak, who has departed the club to return to France.
"It was good to see him play those Test matches at fly-half, but I think it's also important to understand that the Boks play a different style of rugby to us," Plumtree told the club's official website.
"I think the Boks play more off the scrum-half and we generally play more off 10 so he'll have more responsibility with The Sharks. But he started playing really well at fly-half in last year's Super Rugby competition before he got injured, and then obviously Fred came into that role.
"I think people forget that Pat picked up quite a few injuries last year, so he didn't have a great season in Super Rugby from an injury perspective. But hopefully the injuries will stay away this season and he'll be able to get a good run of games in that position."
Lambie has 20 international caps for South Africa.