As the N9/N950 input subsystem is based on QT and the N900 input system is based on GTK, no, its not directly possible to port it.
However, given that Hildon-Input-Method itself is open source, there is nothing stopping someone from writing a new set of input method plugins for Hildon-Input-Method that behave similarly to the N9/N950 input method.
I've learned n9/n950 input method is powered by maliit, and maliit is in extras-devel repostiory..