I want to use some NuGet packages inside Unity. I achieved that Unity finds the downloaded DLLs according to this article (https://www.what-could-possibly-go-wrong.com/unity
There is NuGet2Unity that allows to convert any Nuget package to a '.unitypackage'. I've used it to convert "SpecFlow" and i was able to import the resulting Unity Package. Check out their Examples.
It worked for me with:
dotnet.exe run -n specflow --version 3.0.225
Note that you might need to skip dependecies that are deliverd by the NugetPackage which are already deliverd by unity itself.