It's not possible to by channel set input channels as Pre/Post fader individually in a mix (as expressed in the Hardware Manual Online), but it is possible to get an affect like this using two (2) buses.
If we are setting channel one to be "post fader" on a mix and send all of the rest of the channels on the mixer "Pre Fader," you can A.) use a Pre Fader Aux mix, and send that mix to B.) a Globably post fader Matrix.
What you would do is send all channels but channel 1 to the Aux mix and then send only the Aux send and channel 1 to the Matrix mix. In that regard, you now have the ability to turn channel 1 up and down in the Main mix and it will be affected in the matrix, while all pre-fader channels in the Auxiliary will be unaffected.