Manually, sure, but it sounds like the idea is/was to use an automated approach. I imagine many of the same techniques used by modern decompilers could be applied, and there's probably lots of information in assembly that is lost in machine code (e.g., JE vs JZ on x86). One could also assume some knowledge of the coding conventions used by the organization. Even a tool that could translate 90% automatically, leaving the rest to be done manually, would work out pretty well.

In some ways it reminds me of the project to programatically translate the golang compiler from C to Go. Though I imagine the codebase is quite a bit messier!