Just load any Tuba and adjust the tuning so it goes up +12 semitones (1 full octave). Basically, twist the tuning knob to the right as far as it will go, you'll adjust your Tuba sample to sound one full octave above what it is normally set at.
Since the Euphonium (Tenor Tuba) is one octave above the standard Tuba you'll recreate the Euphonium sound by adjusting the Tuba tuning +12 semitones.
I've been listening to some Euphonium instrumentals and my little trick really does work. I'm not entirely sure whether there would be any adverse effects on the formant characteristics of the sample or whether it would sound cheesy, but my "modified" Tuba sounds exactly like a Euphonium, and I've played the entire range and I haven't heard any problems with the formant or the harmonics and resonance characteristics.
So, until Big Band comes out, if you want to emulate a Euphonium, just load a Tuba and tune it +12 semitones. Dead simple.