How add custom stock tag?

[uitlaber]

How add custom stocks?

 <offer id="1638" available="false" group_id="12345">
        <url>http://example.com/product/id1638</url>
        <price>545.30</price>
        <categoryId>1293</categoryId>
        <categoryId>1345</categoryId>
        <picture>http://example.com/images/1638_1.jpg</picture>
        <name>Товар 2</name>
        <param name="Цвет">Белый</param>
        <description>Описание товара 1638</description>
        <vendor>Название производителя</vendor>
        <model>Модель товарного предложения</model>
        <stock id="Москва">
          <available>true</available>
          <price>500</price>
          <oldprice>550</oldprice>
          <url>http://example.com/product/id1638</url>
          <picture>http://example.com/images/1638_2.jpg</picture>
        </stock>
        <stock id="Екатеринбург">
          <available>true</available>
          <price>540.50</price>
          <oldprice>600</oldprice>
          <url>http://example.com/product/id1638</url>
          <picture>http://example.com/images/1638_3.jpg</picture>
        </stock>
      </offer>