Difficult topic. Generally .NET Framework and .NET Core are incompatible. They target a different set of assemblies (mscorlib vs. System.Runtime) which causes incompatibilities since all usages of types are prefixed with the assembly the type is from.
Starting with .NET Core 2 (currently in preview), you can reference .NET Framework assemblies through an invisible compatibility shim. This allows you to reference the assembly and compile successfully.
It doesn’t guarantee though that the application will run successfully, since .NET Core doesn’t provide all the APIs from .NET Framework. You’ll get PlatformNotSupportedException or MissingTypeException and friends at runtime if that’s the case.