[Tannaton]

I don't see any reason why you could not add a supplementary earth as you describe but my view would it would be not worth the effort - I don't think I've ever heard of earthing issues on the Mk1 XC90 with the exception of the horns at the front.

With earthing, the fewer the joints and connections the better, hence a direct earthing cable to the engine bay, potentially the engine block rather than battery to body, body to block etc. which would be double the connections.