Binding C# SDK in Xamarin.Android

Hello,

I am currently coding an Android app with Xamarin, and I was wondering what the best way was to use an external SDK coded in C# within a Xamarin.Android project, as it seems impossible to reference the Extention (SDK) in the references panel for a Xamarin.Androidv2.2 project like one would usually do in a .NET framework...

Any help would be very appreciated