I believe the title says it all.
It would be nice if it could apply to conventional exports as well (export records script step)
For what kind of files? Excel/Tab? All different kinds?
This needs to be defined per file type.
e.g. all text files can be done via MBS FileMaker Plugin and Text.AppendTextFile function.
for Excel files, you can use XL functions to read in workbook, add sheets from other workbooks and save again. The GMImage.WriteImages function can help to write multi image tiff files. For PDF we have both PDFKIt functions like PDFKit.AppendPages (Mac only) and DynaPDF (Mac+Win+Linux+iOS) DynaPDF.ImportPDFFile functions.
Any text file would be a good start (csv, tab, mer). Other formats like MS Excel would simply make this even more useful.
Retrieving data ...