Build JSON from dynamicaly generated form jquery dForm plugin

Question

I use folowing JSON obj and jquery.dFrom plugin to dynamicaly create this form :

<script type="text/javascript">
$(function() {
  // Generate a form
    $("#myform").dform({
        "action" : "index.html",
        "method" : "get",
        "html" :
        [
            {
                "type" : "p",
                "html" : "You must login"
            },
            {
                "name" : "username",
                "id" : "txt-username",
                "caption" : "Username",
                "type" : "text",
                "placeholder" : "E.g. user@example.com"
            },
            {
                "type" : "select",
                "options" : {
                "us" : "USA",
                "ca" : "Canada",
                "de" : {
                       "selected" : "selected",
                       "html" : "Germany"
                       }
                 }
            },
            {
                "name" : "password",
                "caption" : "Password",
                "type" : "password"
            },
            {
                "type" : "submit",
                "value" : "Login"
            }
        ]
    });
});

generated form :

<form id="myform" action="index.html" method="get" class="ui-dform-form">
<p class="ui-dform-p">You must login</p>
<label for="txt-username" class="ui-dform-label">Username</label>
<input type="text" name="username" id="txt-username" placeholder="E.g. user@example.com" class="ui-dform-text">
<select class="ui-dform-select">
<option class="ui-dform-option" value="us">USA</option>
<option class="ui-dform-option" value="ca">Canada</option>
<option selected="selected" class="ui-dform-option" value="de">Germany</option>
</select>
<label class="ui-dform-label">Password</label>
<input type="password" name="password" class="ui-dform-password">
<input type="submit" class="ui-dform-submit" value="Login">
</form>

How to build JSON obj back from generated form elements with updated values like this :

$(function() {
$("#myform").dform({
    "action" : "index.html",
    "method" : "get",
    "html" :
    [
        {
            "type" : "p",
            "html" : "You must login"
        },
        {
            "name" : "username",
            "id" : "txt-username",
            "caption" : "Username",
            "type" : "text",
            "value" : "morowind"
        },
        {
            "type" : "select",
            "options" : {
            "us" : "USA",
            "ca" : {
                     "selected":"Selected",
                     "html":"Canada"
                   },
            "de" : "Germany"
             }
        },
        {
            "name" : "password",
            "caption" : "Password",
            "type" : "text",
            "value": "mika2048"
        },
        {
            "type" : "submit",
            "value" : "Login"
        }
    ]
});
});