Mongoimport - using --upsertFields to extend documents subarray (not replace)

I have this collection

/**
 * Poll Options Schema
 */
var PollOptionSchema = new Schema({
    option:{type:String},
    option_thumb:{type:Number,default:0},
    votes:[{
        type: Schema.ObjectId,
        ref: 'Vote'
    }]
});

I have numerous JSON files, each with the same documents (about 10000 documents) but each contains diffrent records in the votes array (dont ask why). how can I use mongoimport to extend each docuements vote array and not just replace it with another array?