You say strength is secondary to you yet you want to gain 20lbs of muscle?
You also didn't get very specific for wanting help developing a plan. You said you're in excellent shape,"naturally strong" and need to gain weight. Well how much do you weight, how good is your endurance, and how strong are you "naturally"? The more info you give the easier it is to help with a workout. You could get responses telling you to "lift weights" or do a 5x5 to gain strength but without really knowing where you're at, where you want to go and what your weaknesses are it's mostly just shots in the dark.