Yes, you can.
I prefer lactulose (a non-absorbable sugar) because you can "dial a dose"
10 ml twice a day, 20 ml, or even 30 ml depending on how much help is required. Soft, but not runny is the desired outcome.
I would probably have it separate by an hour or two to the Harvoni just to remove the worry of any interaction. So if you morning dose, have your tablets, then some lactulose at 10 am and then before bed at circa 10 pm.