I'm not aware of a way to do this prior to the build of the dependent projects. The pre/post build steps apply to the project with which they are tied and run before/after the main build for that project. However, the dependent projects all get built before it even gets to the prebuild step of the main project.
I'm not sure if this is a feasible option to accomplish what you are looking for, but I wonder if you can create a dummy dependent project that contains the desired pre-build step, and that gets built prior to any of the other dependent projects.