Can a piece of dialogue be treated as a noun and be made plural?
Absolutely.
Example 1:
"It's past your bedtime," said Jan. "Go to bed."
"But—" started Ella with a whine in her voice.
"No buts," Jan quickly interrupted. "Go to bed. Now!"
Example 2:
"John?" asked Helen.
"Yea."
"Sandra?"
"Yea."
"Jim?"
"Nay."
"Oscar?"
"Yea."
"Well, that's three yeas and one nay," responded Helen. "The motion carries."
Example 3
"That's a fine how-do-you-do!" exclaimed Henry.
"That's the third time I've heard you say that today," said Terry. "I'm starting to get worried. Those fine how-do-you-dos are really starting to add up."