Yes, you can totally do this! All you have to do is add some extra inches to your existing measurements. So if your waist circumference is originally 30″ you can make it 32″ with ease. Obviously, the more you add, the more loose-fitting it’ll be. The areas I would be careful on adding too much is going to be the bustline because this will affect the armhole and top portion of a tank top and we don’t want that to be too big. Definitely, make a muslin after you make your pattern so you can test the fit and tweak it if you need to. Good luck! 🙂