I'm not sure I understand your idea. Gaining pure fat is never a good plan. When you talk about people going from "fat to mass," they're just losing bodyfat and exposing the muscle that was underneath the fat. Going on a hog's diet and adding 30 lbs of blubber does nothing for you in the way of adding muscle. In fact it makes it harder to gain quality mass. Getting fat and then attempting to bulk up beyond that is a recipe for getting hugely fat. Lean people will generally have more favorable muscle/fat gain ratios, assuming they know what they're doing.
You want to bulk, preferably starting from a lean physique. Bulking involves eating above maintenance and lifting weights in an attempt to put on muscle -- unfortunately some bodyfat comes along as part of the bargain. This is what bodybuilders do when they bulk up in the offseason.
If you're having trouble gaining weight, eat more. Figure out your maintenance calories, and every week bump your daily intake up by 500-1000 calories/day, until you're satisfied with your gains. Lift weights, get good sleep, eat the standard 1 gram protein/lb of bodyweight, get some unsaturated fats, and shovel down the complex carbs to meet your caloric requirements.
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