How to put all armature bones under 1 root bone

Question

i have downloaded many arm rigs for my first person project in UE4. UE4 only allows for one root bone when importing. All of the arm rigs i have downloaded have many bones, rather than being children of 1 root bone. I have tried making a new bone, then setting it to the parent but the animation randomly flails and rotates when i try and move the IK handles. I was wondering if anyone knew how i could edit these to allow for 1 parent bone whilst not affecting the other child bones. I will not be moving the root bone at all if that matters.

Bone is the 'root' bone i added

When i try to move any of the IK handles, this happens, after i have added the root: https://gyazo.com/202c9159a6b2081acb82beb3b39c03f1

When i move the IK handles before the root is added, they work fine and adjust properly.

can anybody tell me why this does not work and/or how i can fix it.

Note: i made the bone a parent by selecting all of them, the bone last then pressing CTRL+P and hitting keep offset. Here's a gif of what happens when i dont have a root bone:

Answer 1

Select only top level bones.

Select only bones that have no parent, to be children of the new root bone. As can be seen in the picture fore_arm has been re-parented from presumably upper_arm to the root bone via incorrectly selecting all and parenting to new root.

Suggest, after adding new root bone (before any parenting), select all other bones on the same level, and parent those to new root.

For means of demonstration have added the rigify human rig and deleted the spine root bone, then added a new bone "Root".

Shift-select the bones in same level in outliner, lastly "Root" , parent CtrlP and choose either connected to have children connected to Root without offset, or, probably in most cases, offset.