When you access Roundcube through cPanel, cPanel loads an environment as the cpanelroundcube user. Roundcube is just a PHP application, and it runs as whatever user is invoking it just like any other application you would install on your website.
The files for Roundcube are located in /usr/local/cpanel/base/3rdparty/roundcube - you may be able to add a custom alias to load it from this location but since you haven't really clarified what you're trying to do, not sure how to actually help you. You may be able to get away with changing the username associated with the cpanelroundcube user's ID in /etc/passwd, but I don't know what other problems this would cause and I'm certain cPanel updates will overwrite this.