Would a cast help my accessory navicular?

Possibly: Studies show that when an accessory navicular syndrome is present with inflammation, immobilization is one nonoperative conservative strategy that your Orthopaedic Surgeon may employ to decrease the irritation and inflammation. However, Rest, Ice, Wrapping/bracing, elevation and antiinflammatories may be a preferable option due to loss of function. If severe casting may be indicated see ortho plz

It all depends.: It depends on the problem you are having with the accessory bone. Is it longstanding, a consequence of an injury, etc.