Help is definitely welcome.
Basically, we need to take work from
https://github.com/grpc/grpc/pull/6104 and make sure all the unit tests pass (they worked for me locally a while ago) and that we can build the nuget packages from the projects.
The biggest problem currently is that project.json doesn't support all the features we are using in the .csproj files. Also project.json is a concept that is being deprecated by Microsoft (and they will be replaced by XML based project files but the details are still unknown).
It might be possible to target .NET core from .csproj files (a new feature), but I haven't tried.
So, basically, you can try to make things work by using #6104 and create a pull request with suggested changes (please try to come up with multiple smaller incremental PRs, reviewing and merging a large PR is a big pain).
Jan