Ndengane nearly joined Pirates in January from Bloemfontein Celtic, but the deal failed to materialise for undisclosed reasons.
However, the 31-year-old defender is set to link up with Bucs in January next year, according to sources at Phunya Sele Sele.
Ndengane's contract with Siwelele expires in June 2019.
Celtic CEO Khumbulani Konco declined to comment on Ndengane's proposed move to the Buccaneers.
"I can't comment," Konco tells KickOff.com.
"Please wait. We will release an official statement."
Meanwhile, Mangaung-based businessman Ben Moseme has joined Sinki Leshabane in being linked with buying cash-strapped Celtic from owner Max Tshabalala.
Leshabane has been working towards a deal for the past few months.
"Ben Moseme has also come on board and made a strong bid to buy the club," says a source.
"But still Sinki is a strong potential buyer after meeting with Max last week Wednesday."