Update:
While ticking the box you mentioned made the task editable, I have found a bug which crashes the editor:
Trying to drag a gameobject into the Leader field on a formation task immediately crashes the editor.
I have marked this question as solved by mistake(the button was so green and shiny had to click), so any input on this is still appreciated.
BTW is there a way to undo the solved thingy?
I've tried creating a variant of a prefab and add behavior tree to it(formation for example grid).
The problem is when using the editor I can't edit the task through, it's all grayed out.
Maybe I am missing something and it is not possible in the first place to add behaviors to prefabs, and...