450 Ackley:
I take it you don't really mean a roll-crimp on a brass shotshell hull, rather a normal crimp such as for revolver cartridges.

The easiest might be to step-crown a short length of steel tubing of the correct size to slip over the projectile and rest on the case-mouth. A light rap with something handy, ie piece of wood, will form the crimp nicely. Brass shotshells are quite thin and easy to crimp. I have done this for 16-bore spherical ball in CBC cases, but haven't loaded more than a few cartridges at this stage (gun is currently stripped for restoration). No worries with the crimp when firing, though.