I need to do some actions with incoming messages, for example I want to do regexp search&replace action for each message. Current plugin list don't give me this ability. Is it hard to create new KDE Telepathy Text UI Plugin and add it to list? Maybe some howto or examples exists?

Best bet is to copy and paste an existing one.

Asking on #kde-telepathy if you have questions.

For what you want there is a draft example somewhere in the git branch "filters". It was never merged, I can't remember why.

I know that I can get full sources of kde telepathy, find examples and clone part of them. But this is too hard way for new users. Maybe exist some easier way for write plugin, for example, in Javascript or other script language, without creating C code and compiling full project?
For example, qutim.org messanger support plugins in js and other languages.

If users will can easily write his own plugins for KDE Telepathy, after short time we will see many plugins for different things, this improve popularity of KDE Telepathy application.

It's a trade off; writing a cross language plugin system takes time away from the core developers. It takes a _lot_ of plugins to make up that cost.

That said, there is a long term plan that means the plugin system we'll be split C++ and javascript/QML for UI parts.