Since we already allow fireable as a rare word, it seems logical that sackable should be treated in the same way. Actually both words seem to have some other uses, but I think the usages that generally go with offence (UK) / offense (US) would be the most frequent.
So, sackable will be added as a rare word.
Oops, I did spot the odd instance of the word sackable being used of workers. As early as 1976, the New Scientist quoted industrialist Sir Charles Villiers as speaking of "the old time view that labour was cheap and sackable." I agree that it seems logical that the offence ought to be called something like "sackworthy". Dictionary entries for the suffix -able usually give multiple senses, but I haven't seen one that quite fits these two examples.