https and ssl

HTTPS is effectively HTTP using SSL (Secure Sockets
Layer). SSL merely encrypts the content of the packets
before being sent from the server to client.

Yes you can use ISA server to create a rule to allow HTTPS
to the internet server in question. You can either do
this via the default internet protocol rule wizard
(includes HTTP, ftp and Gopher) and then adapt this to
deny any protocols you do not want used.

If you have an app that needs access, you should consider
a protocol filter rule to allow egress to the server
concerned using SSL.