You can add bones easily at the end of the bone chain for example append a face rig at the end of your “head” bone. Bones won’t affect other bones that are not parented to them. You will need to manually add vertex groups (if you don’t want to re-parent the armature and lose any manual weight painting adjusting you did previously) and do weight painting from scratch for all facial rig bones and clean up the “head” bone weights with changed topology of course.
What you don’t want to do is add bones in the middle of your chain. For example if you add spine bones it’s going to change things in the existing animations for the spine up since their position has shifted a bit. It’s ok when you have the bone chain solved by IK like if you had an arm with 2 bones solved by IK, you could add a shoulder bone and just set up the IK solving with 3 bones in the same fashion and the results would be similar since the IK target bone remains unchanged.
I reckon ideally you shouldn’t probably put much effort into nice animations when you don’t know how your mesh is going to end up like. I don’t know what’s your specific case but generally if your meshes look bad, chances are you got some design issues like proportions or masses that need fixing and you can’t really use the same rig anymore. You’d be likely best off treating the rig and animations as placeholders as well and try to replace it all later on as a whole.
But if you “know what you’re doing” it’s very possible to finish a polished and detailed rig without having the mesh ready.