can I create a new invoice template using an existing template

You'll need to create a new template identical to the existing one, Rhea.

If by "creating a new invoice template using an existing template" you mean duplicating an existing template, this option is currently unavailable. However, you may consider manually recreating a similar template by selecting the New style drop-down button on the Custom form styles page in QuickBooks.

 
Feel free to add a Reply below if you have further concerns.