Can I add a retaining wall to the base of the existing face?
Wood exposed to earth, or even gravel, as you discovered, is a compromise at best. You can hedge your bet by using treated wood. Use wood with the maximum amount of preservative. "Outdoor wood" is not enough, the preservative levels must be at least adequate for ground contact. Preferably wood treated for foundation service should be used. Doing something to ensure any water that does contact the wood can easily drain away is also important, as well as trying to prevent water contact in the first place.

9" of soil does not impart much of a lateral load, so almost any retaining structure will suffice. Still, it would be better to not involve the fence if possible IMO. How far back from the existing structure would depend on what you end up doing to retain the extra earth. You could extend the existing structure, which would obviously have 0 offset. At the other extreme, relying on gravity to retain the earth, such as placing a row of large stones, you would want to see a minimum 45 degree slope between the existing wall and the tops of the new structure.

By using an impervious retaining material like stone or concrete blocks, you can backfill with plain soil, no gravel is required for minimal height walls. Taller structures often backfill with stones to prevent hydrostatic pressure from pushing on the wall. When back filling against wood, do everything reasonably possible to prevent water from contacting the wood, even if it is treated.