SED: insert something after the second last line

In a XML configuration file I need to add a line, in order to not to break the last closing tag. Is it possible to do it with SED ?

The number of line of the whole file can change from a server to another…

Edit :
Some exemple of file I need to edit :

<configuration>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <!-- encoders are assigned the type
         ch.qos.logback.classic.encoder.PatternLayoutEncoder by default -->
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

An other exemple:

  <?xml version="1.0" encoding="UTF-8"?>
  <configuration>

    <property name="DEV_HOME" value="c:/logs" />

    <appender name="FILE-AUDIT"
      class="ch.qos.logback.core.rolling.RollingFileAppender">
      <file>${DEV_HOME}/debug.log</file>
      <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
        <Pattern>
          %d{yyyy-MM-dd HH:mm:ss} - %msg%n
        </Pattern>
      </encoder>

      <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
        <!-- rollover daily -->
        <fileNamePattern>${DEV_HOME}/archived/debug.%d{yyyy-MM-dd}.%i.log
                          </fileNamePattern>
        <timeBasedFileNamingAndTriggeringPolicy
          class="ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP">
          <maxFileSize>10MB</maxFileSize>
        </timeBasedFileNamingAndTriggeringPolicy>
      </rollingPolicy>

    </appender>

    <logger name="com.mkyong.web" level="debug"
      additivity="false">
      <appender-ref ref="FILE-AUDIT" />
    </logger>

    <root level="error">
      <appender-ref ref="FILE-AUDIT" />
    </root>

    <logger name="com.mkyong.ext" level="debug"
      additivity="false">
      <appender-ref ref="FILE-AUDIT" />
    </logger>

      <logger name="com.mkyong.other" level="info"
      additivity="false">
      <appender-ref ref="FILE-AUDIT" />
    </logger>

      <logger name="com.mkyong.commons" level="debug"
      additivity="false">
      <appender-ref ref="FILE-AUDIT" />
    </logger>
  </configuration>