I think you are generalizing LeanGains as IF.
Nonetheless, I am slowly transitioning from dieting with IF to maintenance to slow gaining. My protein intake was already ~1.33g/lb, so I'm really just trying to add in some calories here and there. You might add an egg to meal one. You could add more to your preworkout shake, or add a shake during, or another post. You could add another chicken breast or more rice to your post workout meal, etc etc.
The limit of IF is trying to fit the food all in that 8 hour period. So with that said, I see adding small amounts of food to each meal the easiest solution. You could also try to maybe pulse MAG10 or something between your meals. The last option, that I've considered, is doing 14/10 instead of the 16/8.
With that said, and others have repeated, whatever route you go, you should post and log the details/results