You can copy the surface and you can extend the surface. Each are commands of their own.
First: Set the selection filter to "Geometry"
Second: Select the surface you want to extend (one surface!)
Third: The Offset menu selection is now highlighted... select it.
You can offset by zero (0) but for this case, offset some small distance for now.
Forth: With the selection filter still set to Geometry, Select your first edge to extend... the Extend button now highlights above the offset feature menu selection.
...press and hold the Shift key (not CTRL!) and select all the edges you wish to extend.
Fifth: Select the "Extend" feature from the modeling menu and enter your desired value.
Six: go back and reset your offset to zero.
The extend feature follows some kind of surface rules. The resulting surface may not do as you expect in every instance.
