If you mean that you are literally writing 7-bit ASCII, with 8-bit
chars, and you want this marker char, you want to set your file's
encoding to: "8859_1" see http://mindprod.com/jgloss/encoding.html
see http://mindprod.com/applets/fileio.html for sample code to write
to the file.

Now write char x = 0xff;

or char x = '\u00ff';

to the file.

If you want to generate true EBCDIC files, the EBCDIC encoding is not
directly supported, though four EBCDIC variants are under
a variety of names: Cp500, Cp1046, Cp1047, Cp1148.

Then I would do an experiment to see which unicode chars map to your
desired 0xff.  Compose your file in Unicode, and trust the encoding
mechanism to create the equivalent EBCDIC.