Create the map you want with more than 8 players support, but you just add the first 8 ARMYs (you can't add more in editors, but don't forget to add all the mass and hydrocarbons deposits for them), after you finish the map, tested, all works fine, you add a hydrocarbon in each position of the extra ARMY, save map, test again if you want.
After that, you make a backup of the map, and you need to edit _scenario.lua and _save.lua files.
_scenario.lua you just need to add the extra armies in array and the no rush radius, and make neutral armies one number up than the last human army.
_save.lua, you find those extra hydrocarbons you created, copy/paste any other ARMY array, make it ARMY_9, ARMY_10, etc and use the position array of the extra hydrocarbon for that ARMY and delete the hydrocarbon array, in the end you also need to add the extra armies in those big arrays, and rename neutral ARMY if exist there too.
If you don't want to do the file editions, you can just create the map, send it to me and I'll do the file editions.
After you added extra armies, you can't open the map anymore in any map editor, that's why you need to create a backup of it before editing the files.
All the maps I made can be found here:http://www.supremecommander.com.br/scbmapas
GPGnet doesn't support more than 8 players in lobby, you need Forged Alliance Forever
server to play these maps online.
PS.: I will make this tutorial better tomorrow.