Of course, it is possible to change/convert a Rohloff hub between internal cabling and the EX shiftbox, provided the necessary hardware is acquired and installed. The cable routing might be an issue but I did once see a photo of a Raven Tour (internal cabling and routing for same along the top tube and left seatstay) converted to an Ex shift-box that appeared very successful. As I recall, doing so required the mounting bracket at the hub be "clocked" (rotated) 'round the hub so the cables' angle of approach matched that of the originally intended internal cabling (i.e. aimed at the seat cluster, parallel to the left seatstay). Memory is a bit hazy...
As a data point, when I was shopping for a Rohloff bike a dozen or so years ago, there were several new bikes that came from the factory with anticlockwise rotated EXT box fittings, some pointing upwards behind the centre of the hub and level with the axle, some at some lesser rotation like 45 degrees, some horizontal but pointing backwards rather than the optimum installation of the EXT box pointing forwards with the cables brought along the LH chainstay and dropping down underneath it near the EXT box to enter it. In all cases where the EXT box pointed backwards or up, the cable was brought along the top bar and down the seat stay some way, and then looped around in thin air to give an unstressed cable entry into the EXT box. I thought that a rather vulnerable cable run on bikes intended for the Rohloff's original target market of offroad competition.
This happened, I supposed at the time (the middle oughties), because bike manufacturers were trying to adapt old thinking, or perhaps even leftover frames, to a new technology with new requirements. I thought it was pretty slack engineering by bodgers, but it seems to me likely that Herr Rohloff at the very least did not withdraw the guarantee for such installations, and possibly even approved them or designed them for manufacturers in a bind. (It is not generally appreciated that much of the credit for clean Rohloff installations belongs not to the manufacturers but to Rohloff, which drew and provided free of charge all kinds of peripheral and installation gewgaws for their HGB.) I assumed at the time that the Rohloff pull-pull cable system made these installations possible, perhaps even indistinguishable in "feel" and operation from the obvious optimum EXT box installation (cable down the downtube, along the LH lower stay, into a forward facing EXT box).
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This is about the cable run to some pricey new parts.
If I were faced with the dilemma of upgrading an internal change cable run to the EXT box on an existing bike, I'd grit my teeth, paint some plastic ties bike colour, and run the cable the proper way down the downtube and along the chainstay. One may not intend ever taking the bike offload but equally one could someday be riding with that impulsive person we all know who calls out, "Shortcut on this overgrown animal trail through the forest here," and turns into it without waiting for discussion.
