No, there is no SysLogAppender which would report milliseconds bundled in XP. logback SysLogAppender follows rfc3164#section-4.1.2 where fractions of seconds are not supported.
rfc5424#section-6.2.3 with fractions of seconds was never fully implemented due to lack of interest. you can follow https://jira.qos.ch/browse/LOGBACK-588
MillisecondPrecisionSyslogAppender is more a hack than a robust solution. It follows neither rfc3164 nor rfc5424.
There is a workaround: add a desired timestamp into suffixPattern and parse the value later in logstash (splunk, loggly, etc…).