When you open a non-LiveCycle (i.e. AcroForm) form in Designer, it will prompt you to either use a fixed format (Foreground XFA, and more restrictive), or flowable (allowing the functionality you want). If you are not receiving this prompt, then the form is likely a foreground XFA form. You can confirm this by clicking Edit, and looking at the Replace Artwork icon. If it is not grayed out, then the form is foreground XFA.