exec("Add-Ons/Blah_Blah/server.cs"); can execute scripts, but here's how you use add-ons :
1 : Download the add-on into the Add-Ons folder.
2 : Open config/server/ADD_ON_LIST.cs
3 : Add a new line for it in the .cs file.
For example, if I put in the add-on "Script_KickOnDeath", I would put a new line (IN THE CORRECT ALPHABETICAL ORDERED SPOT) saying :
$AddOn__Script_KickOnDeath = 1;
Changing 1 to -1 disables the add-on. Do not change 1 to 0.
Changing a colorset :
1 : Get the Colorset_blah style add-on you want
2 : Copy the colorset.txt in it to config/server/
3 : Start your server
4 : You're done.