Now. I've solved.
with thist command
pdftk allegato_7.pdf stamp allegatoN.pdf output out.pdf
I obtain what I'm looking for: the content of allegtoN.pdf (made by libreoffice
and exported as pdf) merged in foreground with the allegato_7.pdf, to out.pdf.
is it possible to have the content of allegatoN,pdf only on the first page of