Do it all the time with auto transmissions, have been for years. Just have to take it out of "Drive" and use the manual or sequential mode to limit the upper gear. May still need to use the brakes, but not as much. Newer electronic controlled transmissions that I've used will downshift themselves when going down a hill when using cruise control.
Just be careful on icy roads. There is no clutch to slip/feather when down shifting with an auto trans, dropping the trans a gear can break loose the drive wheels and put you in a skid. Better off using brakes (gently!) on ice.
And for the earlier thread on engine braking vs down shifting - you get more effective engine braking by down shifting.
