Legal Question in Employment Law in California

While your goal is an honorable one, employers do have the right to impose grooming restrictions on its employees as long as the restrictions do not violate protected discriminatory classifications. Growing hair to donate to cancer patients is not such a protected classification.

Perhaps, if you approach your employer with some creative idea that might benefit them, as well as your goal (it could be good publicity for them if they sponsor such a program), they may be willing to make an exception for that purpose. But if they refuse, you will have to choose between your desire to help others and the need to keep your job.