Knockout mappingOptions for ko.mapping.toJSON()-method

Problem

I'm trying to send a modified viewModel back to my server. Unfortunately I have to create my viewModel using ko.mapping.fromJSON() and some mappingOptions, which is working fine. Now I need an easy way to convert the created viewModel back to the original scheme.

Desired result

I want to convert this:

var myModel = {
    simpleProp: "test",
    complexProp: {
        simpleProp2: "test2",
        simpleProp3: "test3"
    }
};

into that (JS):

var myModel = {
    simpleProp: "test",
    modifiedToSimpleProWithNewName: "test2"
};

giving me this JSON:

{"simpleProp":"test","modifiedToSimplePropWithNewName": "test2"}

Attempts

I was thinking that, since ko.mapping.toJSON has a mapping parameter, it is as easy as creating another mapping for this method. But it seems like ko.mapping is actually ignoring my custom property creation:

var mappingOptions = {

    // ignored
    'simpleProp': {
        create: function (thing) {
            return "Changed simpleProps value";
        }
    },

    // working
    ignore: ["simpleProp3"]

};

Further Information

I've created a jsFiddle for you.

Please notice: I've simplified the example removing the model generation using fromJSON:

// created and modified by ko.mapping.fromJSON - with custom mapping!
var myModel = {
    simpleProp: "test",
    complexProp: {
        simpleProp2: "test2",
        simpleProp3: "test3"
    }
};

The result will be:

{"simpleProp":"test","complexProp":{"simpleProp2":"test2"}}

And as you can see, the ignore option will be applied, while the 'simpleProb'/create won't.

Any help is much appreciated. Thank you!

PS: I know that this mapping will not achive the desired result. It was just a "will 'simpleProp' be modified?"-test.