Reading the doc
“Flex’s strength lies in enabling interesting secondary effects that enhance the visual experience. It is not designed to build gameplay affecting physics because it lacks functionality such as trigger events, contact callbacks, ray-casting, serialization, etc. Although it is possible to build these capabilities on top of the core solver, they don’t come in the box. For this reason it is recommended to use Flex in conjunction with a traditional rigid-body physics engine, such as PhysX.”
As far as I know Unreal 4 does not support by default a given physics engine so personally I would have no idea as to an answer to the original question as to a custom branch of the default Unreal 4 engine.
Personally I’m on the don’t know side as I have no clue as to what Flex is or what it does, as to content creation" as it’s not an out of the box feature and might have been a question best asked under C++ programming?