Magento: how do I add an additional multiselect in admin config settings for a shipping module?

In Magento admin custom shipping controller I can include a nice multiselect for selecting countries with the following in system.xml:

<specificcountry translate="label">
    <label>Ship to Specific countries</label>
    <frontend_type>multiselect</frontend_type>
    <sort_order>91</sort_order>
    <source_model>adminhtml/system_config_source_country</source_model>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</specificcountry>

I want to include another multiselect in the admin config settings for my own fieldset, possib开发者_Python百科ly with an extra set of values in their own table. How can I do this?