If you have a replacement device taht can house a hard drive and load it into the optical bay, then you should be able to use two hard drives. Hard drives and optical drives use the same connector to access power and data. The second drive may be a little slower if the optical drive was connected using SATA II rather than SATA III, but otherwise there is no reason for issue.
iFixit has a guide for replacing the optical drive. This can be a good jumping off point for installing the secondary drive. https/www.ifixit.com/Device/HP_ZBook_15