It's currently not possible to directly use an existing recipe as an item in a new recipe. We're definitely considering this for a future update.
In the meantime, here is a workaround. At the moment this workaround is only possible in our "classic" recipe editor on our website (log in using a web browser to http://www.myfitnesspal.com , click "Food" then "Recipes" then "Enter New Recipe" (under the heading "Old Recipe Calculator").
Once you have created a recipe and logged it to a day on your diary, you save that single serving of the recipe as a Meal. Instructions for creating Meals are located here.
This serving of your recipe can now be used as an ingredient in later recipes. Simply search for it by name when looking for a recipe ingredient, choose the correct number of servings, and you'll have access to the recipe as an ingredient anytime you want it.