Well you would need to start with writing a compiler plugin for the .NET compiler framework. However, I am not sure if this is an easy task as C::B is a C/C++ IDE and was designed as such. This there is no MSIL / pre-compiling / whats-o-ever step in the event loop which you might need. (I don't know .NET very well, I try to avoid it whenever possible.)
If you even mean an UI designer for .NET that this might even be impossible using wxWidgets. However, it could be attached as stand-alone tool which tidy integrated into C::B using a well-defined interface.