Many map software vendors like German TOP 50 do not include export functions. Printing, however, is usually not a problem.
I therefore suggest to use the print function for exporting map data. Just print your desired map area into a PDF file (TOP 50 nicely allows to set anchor points for that).
A PDF printer driver will be already installed on many systems (if not, please go to sourceforge and download pdfcreator). Write down the upper left and lower right GPS coordinates of your rectangle as you will need them later in OziExplorer.
You will need to download also two graphics packages: netpbm and xpdf.
Move your PDF print export and all downloaded executables into one directory. After running the following script you will see a bmp file that can be imported and calibrated in OziExplorer. Happy navigating!