The ornamental design for a pole accessory with swivelball, as shown and described.
FIG. 1 is a bottom isometric view of a swivel ball pole accessory;
FIG. 2 is a top isometric view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a front side elevation view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines in a drawing illustrate environmental structure and portions of the pole accessory with swivel ball that forms no part of the claimed design.