New Zealand flew out Kyle Mills and Jamie How to join the squad in South Africa after paceman Shane Bond was ruled out of the rest of the tour.
Bond has missed a number of Tests and ODIs for New Zealand
Bond broke down with an abdominal strain on day three of the first Test, which South Africa went on to win.
He was expected to take around six weeks to recover, while Jacob Oram (hamstring) needed more assessment.
All-rounder Oram limped off on Saturday while bowling and had to bat with a runner on Sunday in Johannesburg.
Michael Papps (stomach illness) and Stephen Fleming (bruised forearm) were expected to recover fully before the start of the second Test in Centurion on Friday.
But captain Daniel Vettori said: "Kyle Mills and Jamie How have been called up but whether
they'll be up to speed for the second Test we'll have to wait and see."
Mills, a seamer, and opening batsman How were in any case due to come into the squad for the one-day series which follows the Tests.
Bond, who took 4-73 in South Africa's first innings, has endured long periods out of the game with with injuries.