I'm afraid there's no such thing - but then again you probably already knew that by looking at the ip phone service developer guide.
You could either do it in a hackish way
Push a Key:Soft3 (assuming the cfwdall softkey is at position 3), if successful, push the first three digits, then push however many you still need until you have virtually "typed" the entire number.
Or do it properly which consists of an updateLine which sets the callforward all element to the desired number (probably preceded by a getPhone to figure out which lines are on the phone).