If you want to convert your normal map to a multiplayer map, you can use the following command in the command line tool (which you can open up by pressing the ` key, or ~)

convert_to_mp <CAMPAIGN_NAME>

By running this command the game should convert your map to the .wtcampmp format, and save it inside the same folder as the .wtcamp file. (documents/My Games/Wee Tanks)