Technically it's doable, if you have a soundcard that allows Analog output and Spidif (Optical) output at the same time, however just because it is do able, it doesn't mean you should as doing so with mess up the positioning of the audio.
If you want to keep the T40 as the left/right channel, and use the Katana as the centre/sub, you would have to use the analog/aux input of the Katana, and connect that to centre/sub channel of the soundcard, and connect the T40 to the left/right channel of the sound card.
IMO, you'd be better off just use the Katana Virtual 7.1 on its own.