You can add additional bones to the metarig after posing. The generated rig will correctly create the controls and widgets.
However at this time Uefy Script does not automatically add extra bones to the primary Epic Skeleton. An exception to this is the facial rig and extra twist bones which must be selected while posing the metarig.
All other extra bones must be manually duplicated and added to the Epic Skeleton deform chain. This is time intensive process but it does work. A video tutorial on how to do this is planned.
Future updates to the script may be able to automate this process or atleast make it simpler, it is on the wishlist.
Uefy 1.2 now support some ability to add new bones in a semi-automatic way. Currently you can add the following samples:
You can follow this tutorial to see examples of how to register new bones: https://youtu.be/2xVF9QFu-Bw
Please note due to the way rigify organizes bones it is not possible to add child bones to new samples using the bone register opertator. In those cases you may still be able to add child bones by manually editing the rig. Example: https://youtu.be/yu0hXxn1lu0