How do you extend a surface with multiple faces?

I wish to extend this surface so that the thickness of this 'cylinder' can vary. It does not allow me to use the 'Extend Surface' option. Why is this? Is it because of all the faces?

I want to extend the surface with the red arrows but keep the surface on the inside with the blue arrow the same.

Is there another way I can do this?

SolidworksParts And Features