How can I make fat-free whipped "cream" with a whipped cream dispenser?

There are any number of combinations of hydrocolloid agents that can be used to simulate the viscosity and other properties of dairy cream without the fat or even the dairy. "Modernist Cuisine" by Myhrvold includes a recipe for low fat "cream" which combines skim milk with l-carrageenan, cellulose gum, and whey powder; as well as a recipe for non-dairy whipping cream made from water, cellulose gum, proplene glycol alginate, agar, vegetable oil, and glycerin flake. Other recipes are less complicated with only a single agent such as agar or xanthan gum

You can't. You can make something that will look and taste like whipped cream but it wont be whipped cream.

It's the same question as how does one make alcohol free vodka. You can. You can make a really nice drink which will produce a better alcohol like high if you mix water and good ghb but it wont be vodka.