you should be able to use the rcm_contextmenu_register_command function, the last parameter is optional, you can set it to the object you want to attach your new option to (the main contextmenu by default) or $('rcmFolderMenu') for example....
rcm_contextmenu_register_command('copy', 'rcmail_copyto', $('#rcmContextCopy'), null, 'after', true, false, $('#rcmFolderMenu'));
There is a bug in the code though, so if you set a value for pos then it wont work, it will always be added to the main contextmenu. I have a fix for this and I will publish it just as soon as I have finished another change I'm working on.