the solution:

First of all, I've updated the iText jar and src files to the latest
(1.4.8)
http://www.lowagie.com/iText/download.html
(the jar that came with the jasperReports download was older)

Then, I've added this code that adds the desired font at runtime:

HashMap fontMap = new HashMap();
FontKey key = new FontKey("Arial", false, false);  //(String fontName,
Boolean bold, Boolean italic)
PdfFont font = new PdfFont("(fontPath)\
\ARIAL.TTF",BaseFont.IDENTITY_H,true); //(String pdfFontName, String
pdfEncoding, Boolean isPdfEmbedded)
fontMap.put(key,font);

JRPdfExporter exporter = new JRPdfExporter();
exporter.setParameter(JRPdfExporterParameter.CHARACTER_ENCODING,
"UTF-8");
exporter.setParameter(JRPdfExporterParameter.FONT_MAP, fontMap);

Note that "Arial" parameter in FontKey method points to the iReport
"Font Name" field (although the font added is actually "ARIAL.TTF") -
so when designing the report, you have to choose "ARIAL.TTF" as "PDF
Font Name" and "Arial" as "Font Name"!!! this is very important cause
for some reason, at run time, Jasper engine searches for the TTF font
by the key of "Arial".

Then, all you have to do is to set the parameters and export the
report (or generate the report in any other way) I've done it this
way:

//@ pathOfJasperFile - the compiled jasper file
//@parameters - a map with report's parameters
//@ conn - a connection to the DB
JasperPrint jasperPrint =  
JasperFillManager.fillReport(pathOfJasperFile, parameters,  conn);
exporter.setParameter(JRPdfExporterParameter.JASPER_PRINT,
jasperPrint);
//@pdfFileName - the file that jasper engine will create. (pdf file)
exporter.setParameter(JRPdfExporterParameter.OUTPUT_FILE_NAME,
pdfFileName);
exporter.exportReport();

notes: a. you'll have to add a specific font for Bold font Italic font
etc. (remember to set the FontName)
b. if, for example, you'll add a font and mark it as Bold - you have
to choose "Bold" at iReport (when   designing the report) or else
jasper engine would not find the font that you've added, even if all
other parameters are right!

Hope this helps.

Dror Friedman

Finally

(THANKS for the help Hermann!!!)