Nothing keeps you from attaching a piece of 14-2 Romex cable to the appliance junction box, attaching a plug to the other end, and plugging in the appliance to the receptacle under the kitchen sink.
But it would be better to wire it in directly. Remove the receptacle and substitute a blank outlet box cover with a single round hole in it into which you put a cable nipple to run the Romex cable through.
But it would be better to wire it in directly. Remove the receptacle and substitute a blank outlet box cover with a single round hole in it into which you put a cable nipple to run the Romex cable through.