British actress Chelsee Healey, comedian Jack Dee and DJ Greg James are also among the stars who will brave crocodiles, hippos and huge rapids during their 68 mile (109.4 kilometres) journey on the Zambezi River.
The group will ride in dugout boats, canoes and white water rafts for the Comic Relief challenge, during which they will be camping on cliff-tops and watery riverbanks.
Chisholm says, "It's going to be tough. It's going to be physically and emotionally demanding, but we've all got each other and hopefully we'll be able to get through it."
The BT Red Nose Challenge: Hell And High Water expedition is being staged ahead of this year's (13) Comic Relief Red Nose Day on 15 March (13), and is expected to raise $1.6 million (£1 million) to help educate children in Zambia.