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. 

1. Open your existing bill.
2. Assign a Customer/Project name or make it billable.
   
3. 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.