Use FBXConverter for model with more than 32K indices?

FBXConverter has a maximum limit of 32K.
I tried converting a model from Mixamo, which seems to have over 80K vertices, with the sad result:

Is there some automated way to reduce the number of vertices so I can use FBXConverter ? or is there an alternative tool to FBXConverter which can do what I need ? (convert the FBX to c3b and c3t)