There is plenty of room for three more wires in the 1/2" EMT. I would install a ground, the original installation is using the conduit for a ground, but it's can be a marginal way to provide a reliable ground. In this case I would push a fish tape from the outlet toward the panel and feed the wires from the panel. This is a good time to head upstream from this subpanel, and KILL the power to the panel. I have no issue doing something like this hot, as long as I have a qualified helper with me, but it's needlessly dangerous for a DIYer. Receptacles for stranded wire have a clamping plate located under the terminal screw, so the strands are trapped between two layers of metal, and then locked down by the screw. They are real common, just find one on the shelf at the big box store and open the box it's packed in to assure that you have the right one. Remember there is no reason to not use solid wire for this installation, it can be a bit more of a pain in the ass to pull, but it's easier to terminate. Finally, there is a pretty easy way to cheat a simple install, if you are dealing with an existing simple conduit run going from the box to a receptacle. Take the receptacle out, tie and tape a string to the exiting wire pair and pull it backwards, into the panel. Now you have a pull string, and you are ready to add a few wires to the group you pull back in. Good luck, and be safe.