Unfortunately, FitDay does not allow for this.
This thread is stickied here in this forum, and it gives you the reasons why (short answer: weight gain is a much more complicated algorithm that is different from weight loss, and the program doesn't support it).
I wish you luck in trying to help your father - I have a grandfather who is underweight and can't seem to keep pounds on after a prolonged illness, so I know how frustrating it can be. Good luck and good wishes to you both.
Female, 31 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/29/14 = 180.2 (net: -35.0)
Current mini-goal: 180 lbs.
Next large goal: 165 lbs. by 12/25/2014
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."