Just in case you're not wanting to extend the curve and you just want to add more points, you can select the curve, right click and select 'Curve Point'. Then hold down the shift button while adding points. Once you've added all the points you want to add, go up to the edit curve menu and select 'Insert Knot'.
Don't be satisfied with what you can do but rather strive to do the things you can't do!
Exceed Expectations!