I don't think it's possible with the current AddOn framework. Or at least, I haven't been able to get it to work after quite a lot of trying, at any rate.
Since Alibre loads the AddOn using LoadLibrary and calling various exported methods, I think a C++ wrapper dll will be necessary. However, this still didn't get it working for me. I think going from managed code (Alibre Design) into unmanaged code (C++ wrapper) into managed (C# AddOn) in the same call caused .NET to flip out.
Though, if anyone has found a solution, I would be very interested in hearing about it as well.