I think that a direct answer to your questions can be: "you can create your own log4j appender" (this is how you can do that). Technically log4j comes with a plethora of appenders and you can always create a new one.
This appender will probably use the Apache POI (link) which provides a Java API for creating and formatting excel spreadsheets.
This is all good, and will technically work but I would think twice before doing this. After all using Excel for logs sounds awkward :)
I can propose a compromise if you still to have an excel spreadsheet.
Use the 'File Appender' to create the CSV file (comma separated value).
The excel will easily read such a file and its good enough for initial processing.
You won't need even to write your appender just make sure you are configuring your layout properly.
Alternatively I can point you on some tools that can help in logs processing:
Apache Chainsaw
Lilith
There is a nice list of log viewers here, I'm sure you can google for more.
Hope this helps