Some chromebooks can support the RW_LEGACY option but many older ones can't. If yours can it is possible to dual boot the machine and retain your chromeOS functionality and have a discretely bootable linux system as well.

You most likely will need more storage to support this kind of setup. Since this is advanced usage, I won't cover that sort of thing here. Since google in their wisdom will stop supporting your hardware/software after only five years you probably don't care about dual booting anyway do you?