As zalo said, the Scene editor is a great way to premake bunkers for regular use.
There are also a couple good mods with prefab bunkers that you can use to just stick into any skirmish map without having to deal with the scene editor.
For example:
L&P Fortifications and
LFoW's Remastered Modules and PrefabsGotcha!'s
Map Pack 3 has some great bunker modules for building with and his
UniTec mod has some larger prefab bunkers ready made. I know the map objects in the MP3 work (at least seemingly) fine in b24, but I have not yet tested Unitec in b24.