There are two ways to do this.
1) Work out the volume fraction of water on the boundary and, using species, add this in and solve as normal. The drawback of this approach is that an extra equation is needed, and this may slow the calculation slightly. Only the RH value used on the boundary is solved.
2) Calculate the solution and save. Turn on species, and patch the correct water volume fraction onto the results. This approach assumes that the volume fraction is constant over the whole domain, and that the humidity has no effect on heat transfer etc. It is also assumed that the vapour mass fraction is constant between all inlets.
The advantage here is that several humidity calculations can be very quickly carried out simply by changing the value of water fraction patched to the domain. Note, neither approach includes the effect of condensation. This will occur where the relative humidity reaches 100%.