Currently, each entry of the neighbour-structure needs to have two fields: 'label' and 'neighblabel'. cfg.neighbours must be a struct-array, with each entry having these two fields. You can then define the structure as follows:

  cfg.neighbours = struct;
  cfg.neighbours(1).label = 'Fp1';
  cfg.neighbours(1).neighblabel = {'Fpz'; 'AFz'};

Similarly, you can load a template and then change the neighbour definition. Note, that you should verify your definition using ft_neighbourplot.