The issue was not that the file I wanted to transform was a .nuspec
file, but that it was not in any project. The Slow Cheetah extensions are applied on a per-project basis, but my .nuspec
file was simply added to as an item to the solution.
Once I added it to a project, the 'Add Transform' option appeared. It seems that adding it to any project would've worked, but I'm currently using the NuGet Packager project template.