Many values in XML value field

  <appender name="LOGSTASH" class="org.apache.log4j.net.SyslogAppender">
    <param name="Threshold" value="DEBUG" />
    <param name="facility" value="SYSLOG"/>
    <param name="SyslogHost" value="" />
    <param name ="loggerFields" value =" ????? "/>
    <layout class="org.apache.log4j.PatternLayout">
      <param name="ConversionPattern"
       value="%d{yyyy-MM-dd HH:mm:ss,SSS} %p %m%n" />
    </layout>
  </appender>

I have a xml file like this, and I have to keep in value of "loggerFields" something like this:

<pair key="THREAD_ID" value="%X{threadId}"/>

I know that "loggerFields" takes list of KeyValuePairs, but i have no idea how can i write it.

What is the best way to obtain this effect?