Alright, what you gotta do to make a map playable in OP2 is:
Find a mission dll that you want to copy
Say you wanted to replace ml4_21.dll, which is the DLL for La Corrida.
All you have to do is open it in a hexeditor, and do a Text Search for .map. When you find it, replace the name of the map file (original map for La Corrida is on4_01.map ) with your own name.
BUT... the new name cannot be longer than the old one, or it will cause problems. If the new name is shorter than the old one, after you type .map, replace the leftover letters with 00 (hex) (use the hex display, not the text display to do this). If in doubt how to do this, just make the map's file name the same length as the old one.
Now save the dll (under a different name that isn't already being used.. just be sure that it begins with ml4_ to recognize it as a 4 player last one standing)
Make sure your .map is in the same folder as OP2, and run the game. Go to multi and run your map (to test it just open two copies of OP2, the first one hosting, the 2nd one connects to the IP that op2 gives).
Good luck!