I really don't understand your question. You are asking for a plural noun that behaves like a singular word? That is a contradiction in terms.
There are some collective nouns which behave differently between British and American English (i.e. The same word is grammatically plural in one langage, and singular in the other).
There are many singular nouns which end in "s" and are grammatically singular.
I know a word that is a collective plural in its singular form, and you can add an -s to it and get another plural collective noun.
Is that what you had in mind?