Hello LizaM1, 

Welcome to the Community page, 

In order to be able to use that employee as a supplier as well you just need to tweak the name slightly so it is not exactly the same so for example if their name is Jane Smith you would just need to put it down as a supplier as Jane Smith1 in order to then save it. Also just to make you aware that the supplier 'display name as' field is for internal use only so the employee wouldn't actually see this as Jane Smith1.