3-Way switch on single pole--convert to combination?

Question

I have a 3-way switch (top and bottom of stairs) to control an overhead light. The whole landing area at the top doesn't have an outlet available anywhere, so my ex-father-in-law (a DIY type who has flipped some houses) confidently told me he could swap the top-of-stairs switch to a combo outlet that would have a switch for the light and an unswitched outlet.

His daughter and I split and the project sort of stayed half-finished--the switch at the bottom of the stairs stopped working, the top switch worked for the light but no power came through the outlet. I finally got around to hiring a pro, and he put it back the way it was (just the switches) and told me he couldn't put in an outlet there because it's a 3-way wired on a single pole, but that it would be possible to DIY it with a GFI outlet on the bottom/3-way on the top and keep the switch working while adding an outlet.

Does anyone have any detail on how this would be done? I'm decently handy but have only done much more straightforward electrical work. I'm not trying to burn my house down, but it would be REALLY useful to have an outlet available in that area of the upstairs.

EDIT:

Answer 1

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 "spot" 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.