Good question, abhinandan.
I'm here to give options on how you can add an existing bills to an individual projects in QuickBooks Online (QBO).
You can either assign a customer name to the line item of your existing bills or make it billable to the project.
- Open your existing bill.
- Assign a Customer/Project name or make it billable.

- Click Save and close.
Just in case you want to learn about Project Profitability, you can check out this article: Quick Update: Project Profitability.
You might also want to visit our QuickBooks Online learning guides for tutorials and webinars about the product.
Drop me a reply if you have other questions managing bills on projects. I'll be here to assist.