scribus 1.4 cannot import a pdf.
but scribus 1.4 can load a PDF in an image frame.
and it even has an experimental feature for keeping the original PDF in the resulting PDF (without rasterizing it)
scribus 1.5 can load a PDF in an image frame (as scribus 1.4 can) and, on top of it, can (try to) import a PDF as native objects (warning: it outlies your text...)
so, if you want to simply add your content on top of a pdf, you don't technically need 1.5.