Binding geometry to skeleton after skeleton is animated

I’ve run into a problem where I need to parent a new pair of eyeballs onto an existing, already animated human meta rig armature with keyframes, so that it moves along with the head.
I’ve tried various ways (in object mode) either parenting with weights, or just to the skeleton object.
No matter which way I try the eyeballs do not seem to parent to move along with the animation.
Is there way to amend an animated armature in Blender?

Well what version of blender? When you use object mode you parent object to object. Not what you want. You want to be in pose mode and then parent the eyes to the eye bone. Did you generate a rig or are you just using the metarig? It would be a lot easier to help you if you supply a blend. Just put your new eyes in it and the armature if you don’t want the rest of it to be seen. You can post here and give us the URL you get to review the blend.

There are two ways you can bind the eyes to the rig:
• as children of the head bone (or eventually of eye bones).
• as children of the armature via an Armature modifier.

in the latter case the eyes must have a Vertex Group with all the vertices selected, whose name matches the name of the driving bone.

For both the methods you must first set the armature in its Rest Position and the eyes in their proper location and angle, then perform parenting.

Thank you for the quick responses and the solution about setting the bones to “rest” position which worked as I needed it to. My mistake was trying to bind the eye rig bone to the human meta rig without the rest position applied and it simply wouldn’t apply the command. The version was 2.8 and it was the default simple human meta rig generated by blender, trying to bind an eye rig generated by Danny Mac’s eye generator plugin.