Add the mods to the client version (game) then run in game console, type "export_server_packages"
Once done you copy over the files created to the american truck simulator folder.
Restart the server.
from a steam post:
Enable Console & Export Server Packages
Go to "C:\Users\%username%\Documents\American Truck Simulator" and open up the config.cfg and edit following:
uset g_developer "0" to uset g_developer "1"
uset g_console "0" to uset g_console "1"
Save file!
Now run ATS and hit the console key "~" and enter following:
export_server_packages or export_server_packages destination file
Wait until its finished!
2 new files has been created in \Documents\American Truck Simulator:
server_packages.dat - contains internal map data
server_packages.sii - contains map details, dlc and mods configuration