3-Way switch on single pole--convert to combination?
Unfortunately, this isn't really straightforwardly possible with your current wiring
Sadly, you've been bit here by one of the all-too-commonly misunderstood things in mains electrical wiring: the old style of switch loop, where always-hot power is brought to the light fixture first, then down to the switch (or switches, in your case), and switched-hot is sent back up to the fixture, without ever bringing a neutral wire to the switch location.  You see, an ordinary light switch never touches the neutral wire.  However, if you want to power something at the switchbox location, such as a plug load on a receptacle, or even something as trivial as a nightlight or smart-switch device, you need both hot so that power can get to your gizmo and neutral to provide a path for that electricity to get back to the utility from whence it came.
It's possible to correct this with by replacing this whole complex with smart-switches that talk over RF or powerline comms to a control module in the light fixture box (I personally favor Insteon for these sorts of &quot;spot&quot; smart-switch retrofits due to its powerline-centric, peer-to-peer design) and retasking the white wire in the cable from the light box to become a neutral since it's no longer needed to serve as a switched-hot, but that's a more involved project, especially since you want to put a receptacle at this location as well.