By 'it' i meant the speedkart gamemode
Uh yes. You have to copy the speedkart gamemode, rename the speedkart gamemode, and manually edit the the speedkart gamemode itself. There should be a gamemode.txt file in there. Edit it, put in 'ADDON [name of your addon]' in a new line.
Get 7zip or winrar.
Do not edit the server.cs. ONLY EDIT THE GAMEMODE.TXT THAT IS INSIDE GAMEMODE_SPEEDKART.ZIP(or whatever you renamed it to). Near where you see all the 'ADDON' stuff, add another line and simply put in the name of the add-on you want to put in.
Duplicate the zip file, rename it to something else, delete namecheck.txt from the archive and THEN make your edits. Default add-ons are replaced by the launcher every time you start the game.
I don't see a namecheck.txt anywhere