I installed the VS2012 plugin and immediately tried to add a new TypeScript file to a WinJs Store app, but it's not possible to add such type of file.

When  this scenario is going to be supported?

Thanks

We do provide a Win8 sample built using TypeScript.  Our project support in Visual Studio is quite limited for this preview release, but hopefully this sample should get you started on how to structure and build a project.   See http://typescript.codeplex.com/SourceControl/changeset/view/fe3bc0bfce1f#samples%2fwin8%2fREADME.txt