You may dynamically add the data (for example the current date) to the header/footer definition teamplate:

Java:

String pageNumberTemplate = getFormattedDate() + ", page $[page]";
footer.setPageNumberTemplate( pageNumberTemplate );

JSP:

<%
String template = getFormattedDate() + ", page $[page] ";
%>

<pd4ml:footer
    pageNumberTemplate="<%=template%>"
    titleAlignment="left"
    pageNumberAlignment="right"
    color="#008000"
    initialPageNumber="1"
    pagesToSkip="1"
    fontSize="14"
    areaHeight="18"/>

The actual versions also allow to substitute placeholders like $[var] in headers/footers or in the document body with the API call:

Map m = new HashMap();
m.put("var", "value");
m.put("date",   getFormattedDate() ); // to replace $[date]
htmlConverter.setDynamicParams(m);