Thanks for the reply!
Unfortunately I cannot open the link as it directs me to the customer support area, but I suspect it refers to the DEFINE_RELAX_TO_EQUILIBRIUM command. I don't think this macro can help me with my issue, since it seems to be only meant to change the value of characteristic time and the equilibrium mass fraction is not an argument in the function.
What I am looking for is a macro to access the cell value of the equilibrium mass fraction, such as C_STORAGE_R(c,t,nv), where nv would be something like Mass_fraction_equilibrium[species_index], or a workaround for this issue. However, I have not found any information on this yet. I also searched the fluent/src footer, but did not find any clues there either.