Create namespace that can be accessed from multiple Unity Projects

I'm hoping to make a set of classes functions that I can use among multiple projects, but I want to be able to only update one file and have all of my projects reference that one file, instead of having to copy and paste the template any time I update it. Is it possible to do a " using "C:\Users\user1\customNamespace.cs"; " or something like that? Or if there's any better way to go about doing this? My backup plan is creating a batch file with a list of everywhere the file is contained and replacing them with the template when you run it. My best solution, but still pretty clunky