Yes, you can load .357 on a Progressive.
You will probably need a Magnum length Expander (shorter). The 38 Special Expander will probably interfere with the powder slide when raised enough to load .357 Magnum. Depends on the machine.
If you will be loading full Magnum loads, you will want a 1/2" powder slide and either a set of spacers or a 1/2" powder slide housing.
If you have a case feeder, you will want an M3A top plate for the Magnum length cases.
You may want a different seating stem for you seating die depending on the bullet.
I can help with all of the above
Bruce Williams
Star Machine Works
PS The powder slides I offer use interchangeable bushings, so I can do pretty much any charge you like.