I update this post.
I also have tried to implement a custom tool which inherits from SelectTool in order to replace the default tool. But I didn't find how to replace this default tool in the diagram.
I've looked at the Diagram and SelectTool classreferences, but the KD article
http://www.syncfusion.com/support/kb/diagram/Default.aspx?ToDo=view&questId=48 seems unavaible.
Thanks for your help.