Tu phrase non ha essite addite perque le sequente jam existe.
Both the English and the Japanese look dodgy to me. Beginning with the English: the 'OK' tagged on at the end of the sentence in question sounds strange. Assuming the rest of the sentence is all right as it is, I suggest substituting ; 'don't they?' for 'OK?'. The sentence would then read: 'With the subjunctive past all the 'be' verbs become 'were', don't they?'. An acceptable Japanese translation of that would be: 「仮定法過去の場合「ｂｅ」動詞はすべて「ｗｅｒｅ」になるのでしょうね。」. Or, you could say: 「仮定法過去の場合「ｂｅ」動詞はすべて「ｗｅｒｅ」になるはずです。」, which I'd translate as: 'With the subjunctive past all the 'be' verbs would (should) become 'were', wouldn't (shouldn't) they?'.
