This is not possible. However, you can create an entity with an attribute that contains all desired values and use this entity. You can create an object of this entity for every value that you need. You can use this entity on for example for an input form, where you let users select an object over association using a reference selector.
Hope this helps!
No: enumerations are defined at design time. The usual solution for this is to create a reference between the entity which would contain the enumeration and the entity which holds your unique names, so that a single value can be selected in a dropdown.