Branden,
It's not possible to add stuff to microflows from runtime actions. I.m interested though what your use case is, maybe there is a different solution for this case.
The plausibility of your suggestion hinges entirely on what you want the split to do. If you loop over the potential reference selector objects you will can model a unique split based on the new object. However, that's only half the battle. I'm assuming that you want to implement unique behavior for each split. If so; how is the modeler supposed to know what 'maybe' is supposed to do unless you model it?
Branden,
Having thought about this for a bit, I think you may need to revisit your model. In my understanding, enumerations are intended for lists of things that are likely to change infrequently, if at all, like states or counties. Things that will change frequently should be maintained in an entity inside of Mendix.
As to being able to perform activities with newly created objects in an entity, perhaps a model like this could work:
Consider the following example:
Just one way to accomplish what it sounds like you want to accomplish.
Mike