Seeker,
The answer is no... you cannot do that through the GUI in SME7. As a SysAdmin I believe this is a good thing. Any time a password travels across the Internet it should be encrypted.
You probably noticed that the server-manager only accepts https connections.
Anyway, if you really must change this you can mess with the /etc/httpd/conf/httpd.conf You may only have to change the redirects, but I'm not really sure.
RewriteRule ^/webmail(/.*|$) https://%{HTTP_HOST}/webmail$1 [L,R]
RewriteRule ^/horde(/.*|$) https://%{HTTP_HOST}/horde$1 [L,R]
Of course, you need to do this within the templating system or your work will be erased, but if you are new it's nice to work directly in the config file and then if you mess up you can just regenerate over the top of your mistakes.
Hope this helps.