On Yahoo Mail - is there any way I can transfer-send a COMPLETE FOLDER
(and not every email separately) to another Yahoo mail account I have?

The easiest way is to Winzip or Winrar all the fiels and then export
them to the relevant email address

Yahoo only offers webmail for free, you would need to write a script
that pulls mails via http. If you have a "Yahoo! Mail Plus" account,
you should be able to use a POP3 client to download all mail off your
account. The Yahoo! Mail help also contains "Backup your email account
or mass-download messages with Yahoo! Mail Plus message archiving."

However, that will get you only halfway there. To "put" the messages
into another Mail Account, you will need to send them to yourself
because Yahoo does not support IMAP or similiar stuff). That will,
however, alter the mails slightly (date and time may not be correct
anymore). A better way may be "pulling" the messages via POP from the
old account directly into the new one (see
<http://help.yahoo.com/help/us/mail/ext/ext-07.html>).

Thanks komp81 and questionmarc. :-)