DynamicField names from SQL value
<dynamicField name="*_s" type="string" indexed="true" stored="true" />

<dataConfig>
<dataSource type="JdbcDataSource"
driver="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost/MY_DB"
user="MYUSER"
password="MYPASS"
batchSize="-1"/>

<document>
    <entity name="article" query="SELECT id, custom_key, custom_value FROM mytable">
        <field column="id" name="id"/>
        <field column="custom_value" name=":::WHAT TO PUT HERE?:::_s"/>
    </entity>
</document>

<dataConfig>
    <script><![CDATA[
            function insertVariants(row)        {
                row.put(row.get('custom_key') + '_custom', row.get('custom_value'));
                return row;
            }
    ]]></script>

    <dataSource type="JdbcDataSource"
driver="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost/MY_DB"
user="MYUSER"
password="MYPASS"
batchSize="-1"/>

<document>
    <entity name="article" query="SELECT id, custom_key, custom_value FROM mytable" transformer="script:insertVariants">
        <field column="id" name="id"/>
    </entity>
</document>

</dataConfig>