You would have to create a new atlas - say something like the MNI_TTatlas and make a new file called CustomAtlases.niml in a new subdirectory with that atlas dataset as an ATLAS entry (see AFNI_atlas_spaces.niml in your abin directory for an example). Set AFNI_SUPP_ATLAS_DIR to that directory. That file will then be automatically read in. You would also modify AFNI_ATLASES_LIST to include this new atlas. Alternatively, just extract the particular region you want and transform that. Then use that as your ROI. Unless you need access to all or most of the regions in that atlas, that would be simpler.

Still, I am not sure if this is particular task is really worth the trouble. The Talairach daemon is very, very approximate. There is no corresponding MRI template, so it is approximate even for the TT_N27 dataset in Talairach space. Transforming that to MNI space will add another approximation. If you can find another atlas that has the region in which you are interested, then that would work better. In which regions are you interested?