Hi Hon Chun,
I believe the one provided OOB is built using EXTJS ("xtype":"richtext" ) and the other Wiki rte used FCKEditor. To configure existing feature you can custamize following . The one you are looking does not exist you might have to build custom plugin http://forums.adobe.com/thread/1071715#4726710
How difficult is it to replace the out of the box richtext editor with FCKEditor / CKEditor?
I do not know & it deponds. However I recommend instead of replacing, build your own new richtext editor component with FCKEditor & use the newly build component instead of OOB one. If you decide to use the FCKeditor then you will have to maintain it's compatibility / functioning with upgrades of CQ because this is not an officially supported feature. I would still suggest you look into making the existing RTE work for your purposes.
Your reply is appreciated. Thanks!