Yes, and it's easy: when entering your journey details on the booking form, you'll see an option to select a return journey using the same locations, simply click it and enter a date and time. You can also add a return later on in the booking process, when you're viewing the journey options.
Already booked one-way?
If you've booked a one-way journey, you can add a return journey from the booking confirmation page or by clicking the link in your confirmation email.