Is there a way to add nuget packages to a workbook that aren't hosted on nuget.org?
Not at this time. You could manually download and extract the nuget package and reference the assemblies manually with #r path/to/assembly.dll.
#r path/to/assembly.dll
@SandyArmstrong Thanks Sandy, that's at least something. I'll give it a try.
This is now supported in our 1.3 preview: https://developer.xamarin.com/releases/interactive/interactive-1.3/