The console is disabled in PST, so you can't spawn any items ingame, and there aren't any excellent editors like Dalekeeper or Shadowkeeper for it either. There are save editors, but they don't allow you to mess with items consistently. You can still use Near Infinity for it, though. If you don't have it already, you can get it from Sorcerer's Place, and you just need to point it to the chitin.key file that exists wherever you installed PST.
For me, however, Near Infinity gives an error when attempting to view/edit the torment.gam file, which is where character data is stored. You can still add/remove items, though, but it's a bit trickier.
1. Open up Near Infinity, point it to Planescape, and click "release dialog.tlk lock" from the Game menu. This will allow you to have NI and PST running at the same time.
2. Start PST, load your save, find a simple non-plot-relevant area, and drop an item on the ground. It doesn't matter what you drop.
3. Press the "location" key ingame to determine the area code, (e.g. "AR0502") and save your game in a new slot.
4. Go back to NI, go to the Save directory, and open up the torment.sav file for your new save.
5. Decompress the .sav file, select the correct area, and open it up.
6. In the upper-righthand corner, there's a list of containers. Open them up one by one until you find the one that has "Pile (4)" for its type and contains the item you dropped in step 2.
7. Go to the Edit tab, and click "Add... Item." This will create a new blank item entry.
8. Open up the new blank item and change the first field into whatever item you want it to be. If it's a charm or something, you can edit its quantity by changing the "quantity/charges 1" field also. For single items like your key, though, just leave it at 0. Don't mess with the flags. The names of the items are all listed in NI, but you might need to search a little. The Tenement Key is "keysybil.itm".
9. You can repeat 7+8 to add additional items as desired. Save and close all the various popup windows that this process has created. Make double-sure to save the area file.
10. Now, click COMPRESS on the torment.sav file you opened in step 4. This is very important, as none of the stuff you did above is truly saved yet. Click "overwrite." Now your stuff is truly saved.
11. Go back to PST, and open up the pile from before, and it should now contain both the original item you dropped and all of the items you added in NI.
I understand it's an awful lot of work, especially compared to BG or IWD, but unfortunately it's the only way I know of to actually spawn items consistently in PST.
Hey, I wanted to thank you for the help. I had a broken save file and none of the other guides were doing it for me.