you would create a macro like this:
mail {
exec(command_to_launch_mail_program_goes_here);
}
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depending on the mail client in question, the command might consist of only the path to the exetuable for the program, or, for some programs, you can add commands to open the new message editor, etc...
for general information on creating a macro and a button, see here:
http://kmeleon.sourceforge.net/docs/toggle.php