AutoCAD .NET :: Modify Object-snap Points For Blocks?

In my application, users will insert predefined blocks which  have simple shapes like rectangle,square or circle. 

Users will connect some polyline or lines to the blocks inserted. My task is that blocks must have snap points only on their center so that the polylines must be connected to the block center point not to other points such that midpoints, end points of the edges.

I wonder if there is a way to override snap points of the blocks  while other entities' snap points do not change.