Is this the same retaining wall from your discussion: Fencing screening assistance? If so, perhaps you could start by showing us some images of what you are working with, including the issue with the I-beam. It would also be great to know what type of modwall you are planning on using. As these are modular proprietary wall systems, we'd likely need to know how they attach to offer advice. Are you planning on attaching the modwalls I-beams over the current retaining walls I-beams?
I look forward to seeing those images and hearing more about your project.