You can maybe ask @jftr (the developer at charge of the Kitkat update) to enable the option in this topic: FP1: Macadamia 1.9.9-pre3 Alpha (Android Kitkat)
But to access that topic, you need to belong to the beta testers group. You can request to be part to that group somehow, but I can’t find right now the forum topic where that was explained… 
Edit: I found it, but there are bad news…
https://forum.fairphone.com/t/closed-call-for-testers-of-android-4-4alpha-for-fairphone-1/19411