Prosecutors had to seek permission from South Africa's new National Director of Public Prosecutions to "centralize" all the charges, Mncube said, so they all could be heard in the same trial at the North Gauteng High Court in Pretoria.
"The National Director has indeed given them (prosecutors) that authority and they will be added," Mncube said. "They are not new charges, they were charges that already existed. They were just in a different jurisdiction."
Pistorius' murder trial is due to start Mar. 3 and the 26-year-old disabled runner faces a life sentence with a minimum of 25 years in prison if he is convicted on the main charge of premeditated murder in the killing of Steenkamp on Valentine's Day.
Mncube declined to detail the charges being added, only saying there were two counts and they covered "the contravention of the firearms control act."
The South African media has reported that Pistorius has twice shot a gun in a public place: One time out of a moving car when he was driving with a former girlfriend and another at a restaurant in Johannesburg when he apparently accidentally fired a friend's gun under a table while admiring it.
Pistorius spokeswoman Anneliese Burgess told the AP that the Pistorius family did not want to comment on "legal aspects" of the case.
But Mncube said Pistorius' lawyers had argued against him being charged and tried in a Pretoria court for alleged offenses that occurred in Johannesburg.
"They are not saying he can't be charged for those, they are saying he can't be charged in a different jurisdiction," Mncube said.
Mncube's National Prosecuting Authority would not comment on any of the details surrounding the two charges to be added, but people who are believed to have been present at those incidents were included in the prosecution's list of more than 100 trial witnesses when Pistorius was indicted two months ago.
Pistorius denies murder and says he shot Steenkamp in self-defense through a toilet cubicle door with his licensed 9mm handgun, thinking mistakenly that she was a dangerous intruder in his house. Prosecutors say he intended to kill her, possibly after a loud argument in the middle of the night.
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