There has been lots of discussions on using the cpanel API so I thought I would post some links on how to find the information
Creating cPAddon Scripts http://www.cpanel.net/docs/cpaddons/
PDF Creating cPanel Addon scripts http://www.cpanel.net/docs/pdf/cpaddons
More cPanel Documents http://www.cpanel.com/docs.htm
This may help with people wanting to create addons.
Also there is already one Cpanel Guide available which is made to integrate roundcube and cpanel this is located http://www.hostgeekz.com/guides/cPanel/64/Install_RoundCube_on_cPanel.htm
thanks alot :)