Question- and thanks in advance for answer.

Does Star-CD allows a user to have a saturated porous media, typical Darcy situation, but then add another dispersed phase as an intruder? This is kind of like a 2.5 phase problem: phase 1 is the wetting, initially resident phase, phase 2 is the new invader phase that is percolating into the porous region by buoyancy say, and the '.5' phase is the solid framework- which would have zero velocity and simply be an inert Darcy drag source.

The aplication is what is called 'sparging' in the groundwater field, where dispersed gases are released into a contaminated groundwater bed and the gases 'scrub' contaminants as they migrate through, but do not displace the resident water phase, only bubble through.

Look at the adapco-homepage, there is a presentation about PEM-fuel cell, including a two phase diffusive flow (gas + water) through a porous media (diffusion layer). I think this could match your problem.