It's a client update. eAthena is server software, so that has nothing to do with it.
The client is closed-source as it's taken directly from Gravity and hex-edited to make it work for private server. It's really hard to make...well...any changes to it at all. Not impossible of course, but you have to be into hex-editing/disassembly and need a lot of time to read through all the complete jibber jabber.
eAthena will support it eventually, it's probably not too much work to get it working.