Would be a very cool feature - even if you don’t use it with UE4.
If you would add it yourself as hint: Make use of that “Shift-S” key and snap your cursor to center or startpoint of existing bones and form there “Shift-S” to place the new added bone in perfect line to your parent bone. Those twist bones should just use “Keep offset” (mark new twistbone and then your lowerarm parent bone and hit Ctrl-P). The lowerarm_twist_01_l should follow the rotations of the lowerarm_l bone but without beeing a parent of any further bone. Its on the same level as “hand_l” (which got lowerarm_l as parent as well) in the skeleton tree. If you take a look at the mannequin you could even see that all bones are kept that way (keep offset vs connnected). You could easier “cheat” some animation poses that would not be that easy possible with “connected” bones instead of “keep offset” bones (while those bones still follow like connected if you don’t grab and move them). Regarding the weights I would recommend to create a copy of the existing vertex group. For example copy lowerarm_l and name it lowerarm_twist_01_l. Afterwards remove some weights of both (lowerarm_l and lowerarm_twist_01_l). It might be easier if you paint with autonormalize on - so you don’t add 1.0 weight to both bones on the same vertex. If one vertex got 1.0 at one bone it should get 0.0 weight at the other bone. If it got 0.5 at one bone it should get 0.5 at the other bone. The sum should always be 1.0 (not more not less). If you copy the vertex group then you would get too much weight at first. That’s why I recommend autonormalize.
Adding a further pose would be nice but you would still have to change it to the rest pose (which requires a lots of steps if you would keep all those handy shape-keys).