It would be useful if a special-value option for numeric questions showed (was activated) depending on whether some condition is met. Is it possible? Maybe we have not found the relevant help item.
Can you show the questionnaire where this situation is required?
This is an interesting case
If you know the possible answer when some scenario occurs, why do you use a numeric question? Have you thought about adding a previous question, maybe a single-select categorical question, and depending on this answer decide if the numeric question is going to be enabled?