First, Polyjuice is only designed to produce the appearance of a single person.
It enables the consumer to assume the physical appearance of another person, as long as they have first procured part of that individual’s body to add to the brew
Second, the final ingredient to Polyjuice is the target’s hair (or whatever). This instantly finishes the potion, which visibly reacts as it processes the final ingredient.
"Now what?" Ron whispered.
"We separate it into three glasses and add the hairs."
Hermione ladled large dollops of the potion into each of the glasses. Then, her hand trembling, she shook Millicent Bulstrode's hair out of its bottle into the first glass.
The potion hissed loudly like a boiling kettle and frothed madly. A second later, it had turned a sick sort of yellow.
"Urgh — essence of Millicent Bulstrode," said Ron, eyeing it with loathing. "Bet it tastes disgusting."
"Add yours, then," said Hermione.
Harry dropped Goyle's hair into the middle glass and Ron put Crabbe's into the last one. Both glasses hissed and frothed. Goyle's turned the khaki color of a booger. Crabbe's a dark, murky brown.
(Chamber of Secrets, Chapter 12)
"Now!" barked Moody.
With all of their eyes upon him, Harry reached up to the top of his head, grabbed a hank of hair, and pulled.
"Good," said Moody, limping forward as he pulled the stopper out of the flask of potion. "Straight in here, if you please."
Harry dropped the hair into the mudlike liquid. The moment it made contact with its surface, the potion began to froth and smoke, then, all at once, it turned a clear, bright gold.
"Ooh, you look much tastier than Crabbe and Goyle, Harry," said Hermione, before catching sight of Ron's raised eyebrows, blushing slightly, and saying, "Oh, you know what I mean — Goyle's potion looked like bogies."
(Deathly Hallows, Chapter 4)
Of course, you could certainly try to add more ingredients, but given the description this would not work — the Polyjuice is complete and it would be akin to adding carrots to your orange juice — nothing would happen.
That, or the Polyjuice would be invalidated (and no longer Polyjuice).
Or (and this is what I think would most likely happen), that piece of toenail would turn into the target’s toenail. And I’d rather not drink toenails, whether apparently my own or (actually) someone else’s...