[visual-studio] How do I "Add Existing Item" an entire directory structure in Visual Studio?

You can change your project XML to add existing subfolders and structures automatically into your project like "node_modules" from NPM:

This is for older MSBuild / Visual Studio versions

<ItemGroup>
   <Item Include="$([System.IO.Directory]::GetFiles(&quot;$(MSBuildProjectDirectory)\node_modules&quot;,&quot;*&quot;,SearchOption.AllDirectories))"></Item>
</ItemGroup>

For the current MSBuild / Visual Studio versions:

Just put it in the nodes of the xml:

<Project>
</Project>

In this case just change $(MSBuildProjectDirectory)\node_modules to your folder name.