What does it say now: not defined or hidden? If is "not defined" then this is OK. The field should show up in your FPM configuration. If it is "hidden", then you would probably need an enhancement.
You can try creating a new enhancement. However, I doubt that it will allow you change the property right there. What you could probably do is create a handler class over-riding the GET_ATTRIBUTES method to enable this field (I'm not sure of the method name; it could be GET_PROPERTIES).