Subgroups and aux mixes do have a compressor, but the compressors have no "Key Source" selection, like the input channels have. Therefore it is not possible to use any internal signal as "ducker". Using an external device would mean to take the signals from the subgroups to the external device and bring it back to the mix via Aux In A or B. This is possible, but does it make sense? Isn't it faster and doesn't it sound more natural to bring the subgroup with the band mix down when somebody is talking and during the whole time somebody is talking?