<< It it there any extrude function?

Yes: not in the Bezier Surface plugin itself but in this fabulous free plugin by fredo6 ;)
Joint Push Pull Interactive
http://sketchucation.com/pluginstore?pln=JointPushPull




By xlrlxi
http://600v.deviantart.com/gallery/
Sketchup is not so bad, isn't it ? ;) (Render: Vray for Sketchup)