If I remember correctly.... in the anime, when it showed Starrk meeting Aizen for the first time, he appeared to already be an arrancar. his face and mask both looked the same and the only difference was his attire, which was that dusty brown cloak. If this is the case.... I think Starrk is the only hollow we've seen who is capable of becoming an arrancar out of their own power.
I may be remembering this wrong but I'm pretty sure I'm not.
As always, comments are welcomed.