How to add flexfields to Individual Compensation Options

We're implementing Individual Compensation for our client and we were wondering how we could add some flexfields on the options so that HR can see and operate on the two fields while associating such options to the employees.

1. One flexfield is needed to indicate if the employee is part of a group so it would be a Y/N kind of field;
2. The other is to determine which currency should be considered for the allocated amount (example: an employee from Italy received house allowance while they're working in UAE, so they do receive it in the local currency, not in Euros).