Thanks all for your genuine and prompt responses.Some of them are really good advises and will use in future for sure.
For the current project lengh of the road is quiet big and time is severly less so I took help from other road design software MX to design the grade extend line and then import it in to C3D which works quiet well.
"...so I took help from other road design software..'
Ouch!....just like running to the arms of another man..
in case you are still interested
there are 2 subassemblies that could do the job (the official way)
check them out
This subassembly overlays one side of an existing road, and adds one or more travel lanes to the edge while matching the existing lane slopes.
This subassembly overlays an existing uncrowned road, and adds one or more travel lanes to the left and right edges while matching the existing lane slope.
"The elevations of the Left and Right Insert Points on the Existing Surface are determined, and the slope is calculated between the two points"
This is what the OverlayWidem Subassembly does. you can set the overlay depth to 0 and only get the widen