The pos_weight parameter is specified per-class. e.g. if the 0th class only has one positive occurrence to a thousand negative occurrences then you would set it to 1000/1, 1000, regardless of the relative occurrences of other classes. It only concerns the relative appearance of a positive value for that class in relation to the number of times it’s 0.
All that adding a class for the no-class case would do for pos_weight is add another column where the weight is approximately 30/70 (i.e. weighting the negative cases more strongly to offset how many positive values there are).
All this is assuming that you wish to weight the classes as if they’re all equally likely to occur within the dataset.