Retargeting Animation + Missing Bones

I am currently re-targeting animations to the Polygon Dungeons Marketplace Pack, I have setup the skeleton re-target poses and everything works fine with the animations with the exception of the character eyebrows drifting around their foreheads. I am assuming this has to do with there being an additional bone for eyebrow animation and this is not in the original animations. Anyone else been through this or can offer a way to counter the issue?