Pb208/Pb206 and Th232/U238 have been added to the U-Pb Geochron DRS. It'll be available in the next release (v4.8).
These ratios are not calculated unless values for "208Pb/206Pb" and "232Th/238U" are recorded for the primary reference material. Note that the value in the RM file must be labelled exactly as above (i.e. "208Pb/206Pb", not "Pb208/Pb206" etc). The value for these ratios also cannot be 0.
If these values are not recorded in the RM file, the DRS will not calculate the associated channels so that the number and order of the output channels will remain the same for those that do not wish to calculate these values.
If you have any questions about the above, please let me know.
When v4.8 is released, please give it a try and let me know if you have any troubles.