Unfortunately, I do not believe this is going to be possible with Turpial, or indeed any client which has not been designed to work with KDE's Global Shortcuts interface.
However, if you are not bound to Turpial then a client that seems to offer exactly what you are looking for is Choqok. It has a similar lightweight interface to Turpial, however it is a KDE application and, as such, supports KDE's Global Shortcuts.
The action you want in this case would be **Quick Post** , and has a default global key assignment of Ctrl+Meta+T (on my distro, at least) which you can use from anywhere within KDE to immediately present the New Post window.