I never used L2Patch. First try to open the extension dll with IDA Pro and look for the strings, if the file is packed, you should load the server and dump the dll and then load it into IDA and see the strings
You can dump from https://x64dbg.com
with the Scylla plugin
this will serve to find hidden configuration variables.
If you want to modify the extension and the dll file is packed, you will have to unpack it or create a loader that modifies bytes at startup or injects a new dll.
PD: It is not an easy task, if you have never done it.
