Hi, I am just over 5 months through my course and I am also on 80 mg/day . Just my opinion but I have been taking my daily dose with at least 3 heaping tablespoons of peanut butter during my dinner (due to the fat solubility of the accutane) and I most certainly have gained weight doing this (which is remarkable considering I am only taking in at most 2000 calories / day and already weigh 100 kg at 1.9 m ). It should be noted that I have abstained from exercise for the duration of the course which may also be contributing to the overall weight gain however I suspect that the odds are good you gain some weight trying to take the daily dose with a high fat content meal.
Well, there are two ways you could approach peanut butter since it is relatively high in omega 6 which is linked to increased inflammatory effects; you could supplement with omega 3's to keep the omega 6 to omega 3 ratio stable, or you could find a different substitute for something with high fat content to eat with your dose. Ultimately I think that regardless of which approach you decide to take, if you are consuming large quantities of fat say > 25g for your dose alone, you will gain weight. BUT, it will be dirty weight.
I wouldn't stack on the fat to gain fat. Accutane increases your cholesterol levels, specifically LDL and VLDL. Which causes silent damage to the walls of the arteries leading increased blood pressure and a $%^&* ton of problems later in life. Having worked on a cardiac floor I know what these look like even at 40 years old as i had a guy who had 2 kids under 2 years of age and he had 100% blockage of his main heart artery suffered a heart attack-lucky to be alive. Physically the guy looked 30 and wasn't overweight.
All you need to do is consume 500 more calories than you burn and you will gain weight.