This seems strange because it takes in an operator, but can only do addition and subtraction. If one was to pass in "*" it would still do subtraction. I think instead you might want to define something that takes in a function.

gpii.textfieldStepper.buttons.alterValue = function (that, operation) {
        var currentValue = that.model.value;
        var step = that.options.range.step;
        // allows operation to be a function or function name
        operation = typeof operation === function ? operation : fluid.getGlobalValue(operation);
        var newValue = operation(currentValue, step);

        that.applier.requestChange("value", newValue);
};

gpii.textfieldStepper.add = function (current, step) { 
    current += step;
    return current;
};

You could also look at model transformations for this, but that might be overkill.
http://wiki.gpii.net/index.php/Architecture_-_Available_transformation_functions