You can sour the beer with a sour mash or with pure cultures in the fermenter. A sour mash is often used to obtain a specific level of sourness, then the wort is boiled to kill the bacteria and stop the souring process. This can add a nice tang or sour undercurrent to the beer. Souring in a fermenter is not stoppable on the homebrew level other than running out of food (as most of us don't flash pastuerize our beer).
So, yes, you can skip a sour mash and add cultures to the primary and/or secondary. Which cultures, when to add, and how much is up to you.