I have had to do something similar. I had to program my own sign in Layout in the Personnel Table to INTERCEPT the FMP login of USER NAME and PASSWORD.
One main reason I did it was to REMOVE employee's from DB when they left the company.
You will still use the FMP method AFTER you handle the required info,
If each employee is issued their own account name and password, You can use Get ( AccountName ) to identify who has just gained access to your database by using that info to look up any needed information on them in a table that includes a field for their account name. Adding and removing employees should not require rewriting the script, but simply maintaining the data in that table as well as their account/password in Manage | Security.
Managing accounts as well as synching them to the table of account names can be managed with scripts and a few text fields so that the user maintaining these need not have full access to your database.