The ornamental design for a walking stickadaptor, as shown and described.
FIG. 1 is a perspective view of the present design for a walking stick adaptor;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof, the left side view being of the same appearance; and,
FIG. 4 is a rear view thereof.
The broken lines in all views are included for the purpose of illustrating a portion of the walking stick adaptor that forms no part of the claimed design.