Hi,
if you import a given PDF file into Designer, it is displayed as background artwork in your form.
Most tools of Designer then will be deactivated.
There is no way to edit this PDF background in Designer, as the kind of PDF you create with Designer is completely different to the one you create with Acrobat or InDesign.
You either can edit your PDF in Acrobat or you have to build a new form in Designer from scratch, I believe.