Mapping two joints to the same node - mecanim.

I am working on a rather long-term project. Recently we decided to make the switch to Unity's mecanim which has made some things much easier. Unfortunately, we created several hundred animation assets before this switch, and we've run up against an interesting dilemma. Our character's spines have 5 joints. spine01, spine02 and so on. Mecanim only allows the mapping of 3 spine joints (including the hips) for retargeting. Does anyone know of a way to either allow mecanim to utilize one or more missing joints, or allow us to map the average of two spine joints to one mecanim "slot?" We're using Maya/FBX to author assets.