As far as I know there is not an SDK.
For this requirement we developed activex control(.NET Visual Class library) and registered it on client
and to be able call this control from ABAP we created a 'proxy' class. This ABAP class
inherited from CL_GUI_CONTROL and it is created using CLSID parameter ( the name of
activex control in windows registry is passed).
This class has same methods as our activex control.
As a detail; this class is just a wrapper for OLE object creation and method calls..
I want to write a blog about this topic someday..