Such requirements are common where you hire new employees or create new customers.
Here, in IT 2 validation, you can add user code in LXPADU01 / 02 include which is used by a user exit function in all MPxxxx employee data maintenance modules, called by transactions PA30, PA40, etc.
The current infotype area is assigned to a structure depending on the infotype; then all user validation, etc can be performed, and any error / warning message can be output.
Display these includes to understand how this is done and you can copy/add additional code as per business requirements.
Because this part of the code is for user code, you do not need ABAP Enhancement, bADI, bAPI, etc, which are to be used for changes in standard SAP programs or functions. ABAP'er skills are needed.