Ok, figured it out - at least partially. This doesn't seem to be documented on the manual's Scenario Editor section, so if anyone has the same issue as me...
When working on a custom map, go to Windows > Objects. Keep scrolling to the right (but carefully as the editor has a tendency to crash when going too fast here) until you spot doors. They're not labelled but they're recognizable as a medium-sized "panel". You know you're looking at one because there are usually only three Objects in this group: the door, the sliding tracks for the door, and the door again in its damaged state (note that this applies twice; there is an Object group for two versions of the Door Object, one facing SE, the other SW).
Unlike other Objects in the editor, this can't be placed at zero/ground level. You have to press "A" (the button to raise elevation by 1 point) and then click on the area you want. This seems to be the only way to place doors; one point of elevation is always required.
Too bad making doors require keys to open is such a hit or miss affair that I only got one right after 30+ tries, and I'm still not sure how the editor sets it up.
Hello there !
Sorry for the late answer ^^... I didn't mess with the editor that much back in the days, but I don't remember having that much troubles, so here are some tips... That said, you're right : lots of aspects of that editor seemed awfully messy or constricted - it was apparently impossible to add new art assets -, which probably explains why it had no success at all. This leads to my first tip ^^...
- Apart from obviously learning from the campaign maps' setup, don't forget about the *.smf options. Lots of premade SuperMacroFiles (combined) elements can be imported from the game - they can be found in the RandomMaps -> GenX (Forest/Swamp/Artic, etc.) folders - and simplify conception a lot. Not only that, but you can save as *.smf interesting elements from any map, yours or campaign missions. So, if you spot a well placed nicely designed door, no need to bother yourself for nothing : just export & import it as it is.
- You're right about the Object procedure to place doors and they really should have mentionned that elevation point detail... It's like the SuperMacro possibilities offered by the editor. It should have been clearly explained in the documentation provided, but that's probably part of the vast hord of botched (or rushed ?) parts of the game. So much stuff is wrong in that manual, has been added afterwards in it's 1.0 *.txt version or even modified after that, in the game files, with no mention anywhere, probably as last minute adjustments... You can tell that the developers were probably (very) hard pressed to finish it. The weirdest part about the editor is possibly that weird asset limitation, in a given Object Set : lots of elements are limited to a given environment - Artic, Jungle, etc. -; but this seemed to exclude even doors sometimes ! For this reason, I remember having troubles to find them in the same Object menu from time to time, just because of a Set's limitations. And that's another reason why *.smf exportation comes in handy...
- You're wrong about the keys, which aren't absolutely needed for doors. Just check the way they're set up in the first campaign mission, with no keys evolved - the front entrance one is a good example -, and you'll understand. It is indeed a matter of linking triggers and reactors, especially if you want your doors to work with Buttons located somewhere. It's quite easy to set 5 different options to open a given door, just by linking those to a single (Door) Reactor.
- Don't forget that there's no need for even setting Buttons (or placing keys anywhere) : as long as you've placed a (Door) Reactor dictating an unlocked status (at the beginning of a mission), any unit will be able to action it by itself, just like a common door in reality. This can be observed a lot in the (random) Industrial maps' buildings.
- Likewise, because any door can be destroyed with enough patience (or firepower !), it's often interesting to duplicate your opening Triggers (and Reactors) accordingly : if you want something to happen when door X is opened, it should probably happen too in the event of it's destruction.
Good luck with your imagination ^^ !