How to change a switched outlet to a half-hot outlet?
Yes, you can.


Turn off the power.
Add a pigtail from the twist-on wire connector to the top "hot" receptacle terminal.
Break the little metal tab between the "hot" terminals on the receptacle.
Mark the white wire going to the switch with black tape or marker, so the next person knows it's being used as a "hot".


The top outlet should be always hot, while the bottom will be switch controlled.



Use a multimeter to verify my assumptions are correct, before proceeding.



If the feeder is coming from the other side, then you'll want to rewire it like this.