How to create a nuget package with both release and debug dll's using nuget package explorer?

I'm using the Nuget Package Explorer to create some nuget packages. I've managed to do so just building a project in Release mode in VS and adding both the dll and pdb files to the package.

So far so good, but when I add the package to another project and try to step into the code while debugging, it will step over it instead.

I understand that I need to build and add the Debug dll and pdb to my package if I want to step into the code while debugging. I'm not sure though how to add these to the package I've already create, which already contains the Release dll and pdb file, which are named the same.

Any thoughts?