Any current NFL assistant coach can be added

Defensive Coordinator, WR's Coach, Assistant to the Offensive Quality Control's Assistant, doesn't matter. College coaches or assistants cannot be added (but as below becoming one doesn't resolve a coach as "NO"). Previous head coaches can also be added (as long as they have an assistant position in the NFL currently).

A Coach resolves "NO" if the coach leaves football for a year or retires

As long as they are coaching at college level or above, they will remain unresolved. A season without a full-time coaching position (or at lower than college level), and they would resolve "NO" (i.e. Lezlie Frazier would have resolved "NO" last year). This does mean, however that a coach can be re-added after a year off. A previously resolved coach can be added if they

A Coach resolves "YES" if the coach is ever hired as a Head Coach

Let me know if you have any questions or suggestions for the market!