You don't need any addon, plugin or script to send a screen capture. That is already supported by WLM nativly.
Simply press PrntScrn to make a copy of your screen (or ALT+PrntScrn to copy a window only) and paste it into a conversation using CTRL+V....
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As for makng WLM using other protocols, that is as good as impossible.
And if I may be so bold, since you looked in the registry to search for buttons, I also doubt you have the needed advance knowledge to pull it off.
It is a bad idea to hack a client which was especially made for 1 protocol. To do what you want you're far better of making your own client designed to use multiple protocols.
That is also what all the other people have done who created multiple IM protocol clients. Nobody ever changed a specific protocol client to use protocols which wasn't designed to be using, since that is as good as impossible and realy wasted effort.
If you're not up to it, you are indeed better of using one of the many multiple protocol clients which already exist... like pidgin...
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