I don't know how you could improve. I suppose you could ask your manager.
As for transferable skills, this volunteer position demonstrates you are consistently able to show up, and work for however many hours. Further, if you have colleagues, it demonstrates a capacity for teamwork. These traits are definitely things employers look for. You could get your manager and even your co-workers to act as references for the jobs you apply for. As for the work itself, sorting is sorting, whether you're sorting clothes, parts, or apples. I don't know what sort of paid work, you're aiming for, so I don't know if the work itself will be of much use to you, but the other stuff certainly should be.