Not as straightforward as it would seem as the end of the tube is sealed and the thread that you'd normally attach the extension to is half way down the existing tube and retains the pump shuttle/foregrip.
Instead of replacing the entire mag tube I thought it may be possible to run a pipe cutter round the tube just in front of the threads and then fit an extension the same as you normally would?

Prposed cut point

Any thoughts?, just as we're off to the US I could pick up a tube quite cheaply

nb. This is a section 1 Fac version so no problems with extending it.