Final answer:
A trash bin is the additional required item for a handwashing station to maintain hygiene by allowing for proper waste disposal.
Step-by-step explanation:
The additional required item for a handwashing station already stocked with soap, single-use paper towels, hot and cold running water, and handwashing signage is a trash bin. This is essential for the proper disposal of used paper towels and other waste to maintain a clean and hygienic environment. Hand sanitizer, hand dryers, and disposable gloves are useful additions to hand hygiene but are not necessary if the primary components of handwashing as per the CDC handwashing recommendations are met.