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Yes, it's possible. I won't give a detailed instruction, but here are the main points:
1) Open the map archive with an archiver.
2) Move all the map contents from the single-mission folder to the multiplayer folder (you can see the exact names by looking at any multiplayer map).
3) Check the paths to make sure they haven't been messed up (most likely, the paths are relative and won't be messed up, but just in case. You need to check all the links in the xdb files that refer to the map files).
4) Look in map.xdb and map-tag.xdb; the map type should be specified there. You need to change it to multiplayer.
5) You can do this by opening the map in the editor and changing the player settings so that it allows both a human player and the computer to play for any faction.
Well, that's it, in a nutshell...