when you have used xml pattern, there is a possibility read that log file and load each event into a
org.apache.log4j.spi.LoggingEvent
Object.
But I'm not really sure if that's what you are looking for. If you could provide some more information, may be it'll trigger people to respond with appropriate answers.
OK, looks like
org.apache.log4j.xml.XMLDecoder
is available in log4j1.3, It might help you to look at the source code.