usepackage{pdfpages}
includepdf[pages=-]{mypaper.pdf}

documentclass{article}

usepackage{geometry,lipsum,eso-pic}
usepackage[export]{adjustbox}
geometry{
  paper = a5paper
}

begin{document}

lipsum[1-10]

clearpage
thispagestyle{empty}
mbox{}% Set something blank on page

% Add top part of A4 PDF
AddToShipoutPictureFG*{%
  AtPageLowerLeft{%
    % A4 = 2 x A5
    includegraphics[angle = 90, height = paperheight, valign = B]{example-image-a4}
  }%
}

clearpage
thispagestyle{empty}
mbox{}% Set something blank on page

% Add bottom part of A4 PDF
AddToShipoutPictureFG*{%
  AtPageLowerLeft{%
    % A4 landscape = 2 x A5 portrait
    makebox[paperwidth][r]{%
      includegraphics[angle = 90, height = paperheight, valign = B]{example-image-a4}%
    }%
  }%
}%

clearpage

lipsum[1-10]

end{document}