Havok doesn't provide volumetric liquid simulation if that's what you're talking about.
I've heared that this would be feature of new version of Storm engine for sub-chaser simulator from Akela (authors of "Sea Dogs" line) But I doubt that they would open source or sell it cheap anytime soon. :sad:
So you're on your own with this...
Btw, check Flight Gear (an open-source flight sim) there's a hot air ballon flight model wich may serve as a starting point.