This question somewhat falls under theory since every body is different, I understand that, but figured I'd ask anyways.
Let's say I've managed to find a diet and exercise routine that fits my everyday lifestyle and I'm happy with my body fat percentage (last time I checked years ago I was at ~12%). I've been working out for around 10 years and I'd say my body has sort of peaked as far as any major body transformation. Now let's say I wanted to add a few lbs of lean muscle mass yet retaining my bodyfat percentage. Is it as simple as increasing my calorie intake (keeping the same protein/fat/carb ratio as before), maybe lifting a few more lbs and running an extra mile or two to keep up with the extra calories?