Dynamic actions' purpose is not for validation nor does it help to validate.
It requires to add code in the PAI (process after input) user exit function EXIT_SAPFP50M_002 for the main employee data maintenance program module SAPFP50M. The function uses the include LXPADU02.
This function code may have been changed in many installations to use include ZXPADU02 which would have been copied from LXPADU02.
If you display the include in txn SE38, you can see how an infotype record is overlaid on to a structure based on the current infotype being read/maintained. Then it would validate and do any processing.
There may be already some code for IT 2001 in your installation.
You can add code to validate the current input record as per the business requirements and give a message to reject or warn. If the message is prefixed with 'e', it means it is an error and user can not proceed to save the record.
Further note, if IT 2001 does not use decoupled infotype framework (DIF) in your installation, then it would be more economical to create a function or routine (or form in SAP ABAP terms); and it can be used for both R/3 & ESS/MSS. Else, you'd need duplicate code for these two environments.
Similarly for process before input (PBO), function EXIT_SAPFP50M_001, in include LXPADU01 (or ZXPADU01) applies.