Yes it is. Then I use a standard PC cable to connect to another flash without hot-shoe.
Sometimes I use two slaves, connected in paralel to trigger 3 flash units. All connected using cables and PC connector, no hot-shoe.
You might want to know that most studio flashes have built-in slaves, and no cable connection is then needed to fire them. Your on-camera flash can be used to trigger the studio flashes as long as they don't pre-flash.