adding a form as a layer

Our accounting department would like to go paperless. We now have all of our vendor sending pdf invoices. Our accounting department has some information that needs to be added to each invoice. What we would like to do is add the, for lack of a better term, stamp to the invoice. We have the pdf, which is about 2.5 in x 3 in with the information as a fillable form. The problem is, when we add the form as a layer on top of the pdf invoice and save it, we loose the fillable form fields. When the person who needs to approve it opens it, they have to print it out and fill in the information. Or the Accountant has to create the fillable form on each invoice - which is not possible as she wouldn't have time to do anything else. 

I'm including the approval form that we would like to place on top of the invoice as a layer and the invoice with the form already added. You will see that once we move the form into the file as a layer it is no longer a fillable form.