Exactly WHICH kind of switches are you wanting to cascade.
If you mean cascade a larger external switch (like a 4x8) to a phase III dish w/the built in switches, you need one that specifically says cascadable, like Casey said.
Another way he might mean, would be to cascade one switch after another, like to get a couple more outputs with an existing one you have on hand. In some cases, you can - for example, I have a phase I D* dish, where I have have one 2x4 on the 101 line & another 2x4 on the 119 line. I use 2 of the 119 outputs to download E* receivers from 119 only, & I use 2 of the 101 outputs to go to a D-Tivo.
I then took the other 2 101 outputs & the other 2 119 outputs, ran those into a D* 4x4 switch that I had on hand that I wasn't using, which gave me 4 101/119 outputs, which was enough for my needs.
Only problem I will have, is if I want to combine D* 110, I'll need to run the 5th LNB coax all the way into the house & combine it between the 2 switches, which is no big deal for me.