FWIW, I just ran some .357 cases into my RCBS .357 Seater 1970 die and it indeed makes some beautiful roll crimps, just like my Star.

Unfortunately, the roll crimp ring is too deep in the die to affect .38 spl cases. It does reinforce, however, what others have said ... the RCBS .38 spl Seater will give you the roll crimp you want.