When creating a NuGet package, can I modify existing source code files?

I am trying to create a series of NuGet packages for local use in my organization.

I know that I can create source code conversion files (i.e. MyModel.cs.pp ) to help with adding new source files, i.e .:

namespace $rootnamespace$.Models {
    public struct CategoryInfo {
        public string categoryid;
        public string description;
    }
}

But what if I want to modify an existing source code file? Is there any way to do this? Perhaps using PowerShell?