I have created a curve and used a circle to extrude along it and create a surface.
I want to now add thickness to the inside of the 'tube' that has been created.
Is that possible without converting to polygons and if so how...?

The way I thought I would be able to do it is to select the circular Isoparm at the end of the tube and then create another circle and use the loft tool, but this doesn't want to work and just creates a very ugly face capping the hole rather than adding thickness.