I want to add a W-9 form to my form

THERE IS NO WAY TO IMPORT ANOTHER FORM LIKE A W-9

If you have a W-9 form in a PDF format you can display it within your form using a PDF Embedder widget.

Related guide:

How to Display a PDF Document on Your Form

If you need any further assistance, please let us know.

I tried that but the fields are not editable
...

Unfortunately, there is no way to import another document into the form to make to use it as a native form element. If you would like to accept some data, the form fields should be added manually using available form elements.