Thank you, @Sam-I-Am. I was just afraid that it'll have to be scripted to work but it's a relief that it isn't the case and there are actually various ways to achieve this.
Initially, I did it from the Employee record like you described but the users will only have access to Employee Center role and it seems that it's not possible to expose a custom field in the Employee form available in the Employee Center.
It sounds like I'm going to do the custom record route linked to the Employee record. So, I will have 2 scheduled workflows then, one that updates the Employee record based on the custom record and another that updates the existing POs based on the Employee record.