Visual Studio 2017 for Mac is actually a different beast from Visual Studio for Windows. As you may know, it is originally based on Xamarin Studio which on its turn is an extended form of MonoDevelop.
Simply put, the extensions you'll find in the Visual Studio Marketplace are not compatible with Visual Studio 2017 for Mac.
VS2017 for Mac does have its own marketplace, which is accessible from the Extension Manager. You can access it from inside the IDE through:
Visual Studio menu > Extensions
In the Extension Manager, you'll find a pretty small list of available extensions. But a neat trick is to look at the source of the repositories, which is simply the old MonoDevelop catalog http://addins.monodevelop.com/
Wrapping it up, you can add third party extension url's or install community packages through a file, but basically put there is not a lot more that you can get with the same experience and broad offering like the 'regular' Marketplace.
