I am reusing a rigged model I got from Blendswap. It is fully rigged (by rigify it seems) with the exception of the fingers. The rig comes with controls and shows no actual bones (which I judge would be easily extendable). It also has a complete armature ref bones that is already scaled to the model in a different layer. I took this armature's hand bones and skinned them to the model. The finger then seemed to work/pose ohk but the problem is that I cannot get them to follow when posing the model using the main rig controls (which yield weird deformation when posing the fingers after). I Googled and tried out what seemed to be solutions (like Child of bone constrains, Ctrl + J, ...) to no proper avail. Any assistance will be appreciated. Happy to provide more info that may be deemed needful toward offering the desired assistance.
I Googled further > found that I can select the main rig/metarig > go to Edit Mode > Go to Object Data > scroll to Rigify Buttons > Add two palms bones and the 10 fingers (by selecting each and clicking Add Sample) > scale them and link the joints respectively after. Doing this now and expect it will be all I needed. Just wished I could mirror and thus cut the work in half :)
Thanks for answering the question because I also had the same problem. For your problem with mirroring you could rename the sample(in your case palms) to (the original name.L) and symmetrize the bones and you could work with them like other parts of the rig.
And thank you for your answer you don't know how much it helped me. And yes I'm a bit late to view this post.