There are two ways that you can accomplish this:
1. If you want all of your tasks to execute within FixedUpdate you can manually tick the Behavior Manager within the FixedUpdate loop.
2. If you only want a single task to use FixedUpdate you can override OnFixedUpdate as you mentioned, and then store the results in a private variable. You can then return these results within OnUpdate. You'll need to do it this way since the rest of the tree updates within the Update loop.
Ah, yeah, with conditional aborts from OnUpdate you could call OnFixedUpdate to update the logic. A regular OnUpdate is running in between OnStart/OnEnd, and the conditional abort is occurring when outside those methods.
OnFixedUpdate is within FixedUpdate. If you call OnFixedUpdate within OnUpdate then it will be within the Update loop. There isn't a way to have conditional aborts run within the FixedUpdate loop unless you tick the entire tree within FixedUpdate.