You can create a cylinder primitive object and set it to 6 sides (remove Phong tag for sharp edges), or you can create an N-sided parametric spline and put in Extrude NURBS, as you did with your Illustrator path, or you can take that 2D disc, make editable, select all polygons and use extrude command. But you should be able to edit your original caps if you choose the proper mode (point, edge or polygon mode).
C.