I need to add a new receptacle to a room. The only convenient source of power for that receptacle in behind a light switch. The challenge is that the aforementioned switch is located on the end run of an existing split receptacle. So there is only 1 cable running into the switch box, coming from existing receptacle, and white wire is marked as hot. The existing receptacle is too far to be a viable solution to wire from there. My only option is from switch box. Am I out of luck?
That switch box contains 2 hots and no neutral. So without running any new wire, your only option would be to re-wire the outlet so that it is no longer switched and use the switch box as a junction box.
Here's a before and after diagram. Note that the "neutral" between the switch and the outlet is actually your white wire that has been relabeled hot.
Thinking about it again, you could make your NEW outlet switched in this configuration.
Oliver, the short answer to your specific question is YES, you are out of luck. The previous discussion is viable, but requires rewiring in the switched outlet to turn the conductors going to the switch back to the normal hot/neutral/ground configuration for use with a receptacle , and will result in no longer having a switch and original switched receptacle. Also note, if you do decide rewire the switched receptacle as a source for another receptacle, you may want to replace the actual receptacle because the bridging tab will have been removed if only one half of said receptacle was switched. Not necessary if both top and bottom are switched together. This is rare however.