You can use the same form.
What you want to do is put the form is the document property of a Document Form variable and use that variable as the form data mapping for the user step.
Something like /process_data/myDocForm/object/document = /process_data/@docFromEmail.
Before you do that though, you want to make sure to add the "Inject Form Bridge" operation which will add some JavaScript to your PDF so that it submits properly within workspace.
So to summarize, make sure the document you get from the email is in a "document" variable. Pass that document to the "Inject Form Bridge" operation (look at the RenderPDFForm from the Sample-Forms for an example of usage). Then put that document into the document property of a Document Form variable. Use the Document Form variable as the Input and Output data for the user step.
One note. There is a URL property you can configure on the Document Form that allows you to browse for a file in the repository. You might have to specify a file there. I remember a bug where if the URL parameter is left blank, it causes an error. In your case you don't need a URL, since you pass your own PDF, but you might still have to specify something in there just to avoid the error.
Jasmin