Assignment Fields & Fast Formulas in Compensation

Hi all,

We have a customer who wants to create potential salary increases for employees. Of course, we would like to do this in the assignment and not as a flex field. We would then like to transfer this data to the compensation plan. Does anyone know of a way to maintain such data as standard?
In addition, there are fields such as “Seniority date” that are not displayed as standard in a compensation plan. Is there a reliable solution for transferring fields from Core? 
Is this only possible using Fast Formulas? How many Fast Formulas can be used, or is there a limit on their use in compensation plans?