Adding solution-level items in a NuGet package

I want to add solution folders and solution items (not projects) to a solution file via a NuGet package. I imagine this would be accomplished through Powershell. I've looked through the documentation for NuGet, Powershell, and EnvDTE and can't figure out:

1. Which commands/methods I would use?
2. Which standard script I would do this in, Init.ps1, Install.ps1, or somewhere else?