It is not possible to send the main mix to the auxes. Normally you would use a matrix for this kind of signal patching. The new SL series III has that feature. What you can do is to use the faders of your stereo aux, set the auxes to postfade and all faders to Zero (U). The faders on the main bus then "mix" what you hear in the auxes. Remember to set the pans manually in the auxes. You can also send effects A-D to the auxes. The "copy" of the main mix can be as good as the original, but you will have to rehearse a little bit...
Another "trick" would be to use the solo function of the RM (button "Main" on the front panel) and send the headphone signal by an adapter cable to two of your input channels. In order to prevent feedback you need to deselect the "Main" and "Mono" routing of the two input channels from the main bus. If you don't use the headphone out otherwise, this is a way to get the main mix to the auxes.