How can i add an outlet to an existing combination light switch?

No you can't. You have your always hot white wire, that should be marked with black tape, energizing the switch. You then have the two load wires, black and red, coming off the switch to control two devices. You are missing the neutral that would be required to add the outlet.

If you can combined the two devices at the fixture and connect them to the red wire, connect the hot to the black and the white to the neutral all at the fixtures, then replace the combination switch with a single switch. Connect the black, hot, to switch and pigtail to outlet. Connect red to switch and connect white , neutal, to outlet... just a thought.