Not with native FileMaker capabilities, You'd need a plug-in.
"Save as PDF", in a script can append the PDF being "saved" to an existing file, but not the reverse.
You can use MBS FileMaker Plugin to merge PDFs.
Either by using PDFKit (Mac only) or DynaPDF (Crossplatform).
You can download the plugin and try it with the example databases.
Thank you, Phil and Christian. If I insert (through a script) an image in a global container field I can append it; the appending file shows (like a screen shot), but if I try to insert a PDF or any other foreign file, it doesn´t work, although in the PDF I can read the file in the container field. I guess I have to go with a plugin. Thanks again.