Can I make my own kernel extensions on El Capitan? I have recently heard that a feature called rootless that is going to be on El Capitan. This means that folders such as /System cannot be modified by the root user and only signed kext (kernel extensions) can be installed on the computer.

I have not thought about it before but now I have I would like to make my own kext extensions. The only issue is I am not a paid developer so therefor I cannot sign my kext extensions.

When I upgrade to El Capitan would I be able to make by own kexts and install them on my system while still being able to boot my computer and not having to join the $99/Year Apple Developers Program?
Rootless can be completely disabled on OS X El Capitan (though not on iOS, for hopefully obvious reasons). Therefore if you don't mind foregoing the ostensible benefits it provides, you can essentially revert to Yosemite's security model - which is to say, unsigned kexts can be used, but it requires preparation to do so.

It may not be able to be disabled in future versions of the OS, but for now it can be turned off.