I'm not aware of a direct way to accomplish this but you could use the SP Object Model to synchronize the items in a choice field  with items from a list. Choice and lookup  fields are implemented quite differently. The choice field actually stores the selected value (or values if formatted  to multiple  selections) as a string while the lookup field only stores a long pointer back to the original list  item. This creates another significant difference between the two field types: a value selected from a choice field will not change  even if the selected choice is renamed or removed from the list while the a lookup field always reflects the value that is stored in the list so if the list item is renamed the new value is what appears in the property. Hopefully this is helpfull and I didn't ramble on too far from the question.