the only way I can think of is to uninstall NuGet Package Explorer, remove the association with the .nuspec extension and then reinstall NuGet Package Explorer.
Regarding the Visual Studio integration, that's an interesting idea. However we're probably not doing that in the near future because there are other more important features to tackle. Feel free to open an issue though. If we see enough votes from the
community, we may do it.
Having said that, nothing prevents you or someone from implementing that as external VS designer extension. It doesn't even need to have dependency on NuGet. I can see this as a perfect community project.