I have a mail server running Postfix & Apache for web mail application. I followed this guide which walks you through creating 'self signed SSL certificates for Postfix and Dovecot. The SSL certs are working fine since I tested them with TLS / SASL via email however my question is can I also use the same generated SSL certificates to make my webmail session via Apache secure?

My DocumentRoot is configured to take you to *mydomain.us* and then there is a link for *mydomain.us/webmail* and the 'webmail' sub directory is what I would like to be running on port 443.

Anyone know if this is possible with out some crazy configuration modifications? I would think I simply need to add a 'virtual host' entry in /etc/httpd/conf/httpd.conf file pointing to the location of my SSL certificates on the server.

What you need is a standard configuration of SSL in Apache. The exact steps for doing that depends on the distribution you are using, but I believe it would be somthing like this:
1. Install mod_ssl, if it's not installed already
2. Add or change some global directives in httpd.conf for mod_ssl
3. Add some directives in the virtual host for *mydomain.us* which points to your certificate
4. Remember to add "Listen 443" somewhere in httpd.conf

I don't know if the certificates you created for Postfix and Dovecot can be used. The certificate must have a subject that matches your domain (that is, the common name in the subject must be "mydomain.us" if the certificate should protect that domain).