Unfortunately, for technical reasons, it is not at present possible to edit the current port menu, though it is certainly one feature that we would like to improve. In one or two cases whole countries are currently unavailable.
In the meantime, please use the nearest available port, provided it is sensible (as we both do with Sas van Gent for Sluiskil). If there is nothing, then it is important to include it in the Photo Details box.