"Haven of War" is an example of an SC:FA map with adaptive spots:
This 3v3 player map has 4 metal spots for each player spawn location.
Unless there aren't players for that location.
Then there will be only 2 spots.
Do you know other such maps? We are desperately searching for some.
Is this a feature that is built-in for map designs (but rarely used of map makers)?
Or is it something that has to be done "made by hand" via scripting?
Or what would be the easiest way to make an existing map adaptive in the above sense?

Done via scripting. It isn't terribly difficult to do, just a little tedious.

there is a tool called resource manager tool (or something like that) to do this.

Thanks.
Found it:
http://supcom.wikia.com/wiki/Resource_Management_Tool
viewtopic.php?t=13978

I guess this is not possible, otherwise it would probably already have been done, but could there be a mod that automatically adds this feature (adaptive) spots to a map, even when the map has not been designed with this functionality in mind?

This mod would need to execute a script at start-up of the game, and the script could for example do the following:
Iterate over all metal spots on the map.
Check whether the spot is within some range of an army spawn point.
Check whether that army has spawned for the game.
If not, then disable that spot.