This isn’t possible and definitely not something you want. Because when you could add an attribute to your domain model what about:
in other words, what you are trying to achieve is something you shouldn’t even consider, as it is related to app development, not to a user scenario.
My guess is that you want to add a value to a certain object. Like a car has many characteristics and you want to add one or more dynamically. In that case I would add 2 entities to the domain model. One entity Car, another CarCharacteristic with a 1-* relation in between, so that you can add a list of CarCharacteristic to the Car.