You have a serious underpinning / support of excavation project. I suggest you call one or more leading underpinning contractors and one or more major geotechnical consultants, all of whom have experience with similar projects. I can't help you with names of companies in Cape Town, SA. Sorry.
The bigger problem may not be supporting the existing footings as much as supporting the lowest existing floor slab which is probably a thin slab on grade. A similar project, I think, is being constructed at the United States Capitol Building in Washington, DC.