I'm aware of hidden emoticons of skype, but I'd like to create my own emoticon.
Is that possible to do? If so, how can I do that? Is there any tool for that?
No, I'm afraid that's not possible read this thread.
An exerpt of the same thread, which is a reply from Skype's Staff.
Here is another article for your reference.
This is pretty weak. Mac users are allowed to modify their Skype emoticons.