You could do this:
code:
function OnEvent_ChatWndSendMessage(ChatWnd, sMessage) {
sMessage = sMessage.replace(/:\)/gi,'=)');
sMessage = sMessage.replace(/:\(/gi,'=(');
sMessage = sMessage.replace(/:P/gi,'=P');
return sMessage;
}
The first being the emoticon you want to change and the second the custom emoticon you want to replace it with
.