PHREEQC allows mass-action equilibrium between a phase and the aqueous phase. You can write a reactionCO2(l) -> CO2(aq)so, if you have an equilibrium constant for that reaction, PHREEQC can calculate the equilibrium distribution. That said, you also need to stay within the limits of the phreeqc.dat and pitzer.dat databases, liberally 0-200 C and 0-1000 atm.
All you can do is have a pure CO2(l) phase, analogous to a calcite phase. You could define CO2(l) in PHASES, and include CO2(l) in EQUILIBRIUM_PHASES.Sorry, you cannot have an aqueous phase that is pure CO2. There is an implicit assumption that H2O is the solvent, and numerical issues arise if the amount of solvent is too small. You can always add CO2 to a solution with REACTION or KINETICS (or with EQUILIBRIUM_PHASES).