The property is job.JobExtraInfo0 and you should be able to get and set it. After settings it, you must call RepositoryUtils.SaveJob(job) so it sticks.
A curiosity that should be mentioned - the Job property .JobExtraInfo0 can also be accessed as .ExtraInfo0. This is true for almost all Job properties, e.g. job.JobMinRenderTimeSeconds and job.MinRenderTimeSeconds are equivalent. An exception is job.JobId which cannot be shortened to job.Id.