How much of this were you planning to add to your juice? Adding a little KSorbate won't kill the fermentation if the concentration is low enough. The yeast will absorb into themselves any that is there, and those yeast cells will loose the ability to bud (create more yeast cells). But if the concentration is low enough, there can still be enough yeast that won't be affected to get the fermentation done.
The biggest problem is that it is impossible to know how much KSorbate has been added to the commercial juice. So if you use it, it will be a guess. That said, you should be able to get away with adding a gallon of the store bought juice if you have at least 4 gallons of your home pressed juice.
Pitching a higher amount of yeast up front will also help.