I would like to add extra documents (like reference letters and certificates) to the moderncv in a way that everything is in the same pdf. I can´t include them as text because they will actually be scanned copies.
Is there any way to add it so every scan occupies an extra page in the same pdf or has the template been created to just have the CV and cover letter?
The moderncv document class can handle this just like any other document class. Easiest would be to include
\usepackage{pdfpages}% http://ctan.org/pkg/pdfpages
in your preamble and insert
\includepdf{<file>}
wherever you want to include a PDF <file>. <file> can be multi-page (or not) and you can specify the pages to include. See the pdfpages documentation for options or choices.
pdfpages inserts the pages as-is. You can use the pagecommand={..} key-value to add commands to be executed with every page that is included via \includepdf. Some people use pagecommand={\thispagestyle{plain}} to add a plain page style (and therefore page numbers at the bottom).
empty. Try adding \pagestyle{plain} to your preamble, or load the fancyhdr package and insert them in the usual way...
attachfile(ctan.org/tex-archive/macros/latex/contrib/attachfile) which allows to make a link out of for example a word ("Attached: Cover Leter") and which opens an attached file. You generate the PDF but it includes attached files.