I don't think fitday will do it. I haven't tried to gain weight using the fitday tools. I'm guessing fitday will chart your progress as you gain weight, given a goal to lose weight. It just won't let you have the goal to gain weight.
Perhaps this has to do with the way fitday gives you some forecasting help and small goals to lose weight (not gain weight). It will tell you that to reach your goal in a certain time frame, you must lose 1.2 lbs a week, for example.
But if you ignore the goal setting and just set your goal date and start following your own program to gain weight (more fat in your diet, for example), I don't know why fitday wouldn't just make a chart to show that you are indeed gaining weight when you weigh yourself and input the data at intervals, while you still have a goal date.
Fitday won't give you props for meeting your calorie goal for the day (you'll always go over that goal) but so what. If a visual graph is enough for you, then you have it.