This can be done easily through Modifier as you stated. As for the Field Level Security, the following actions only can be applied on fields;
- Password before
- Password after
- Warning before
- Lock field
- Disable field
- Hide field
- Password window
- Disable window
- Password window
- Password form
- Disable form
Having the field as required is not included as one of the available actions.
Other ways include VBA script.. Though, if your target is only to set a field as "required", Modifier is your best option
Best Regards,
Mahmoud AlSaadi