I have found this in a forum...
Originally Posted by Athens View Post
For adding a new playable faction
1) open the Scripts subfolder in PON
2) Find the file named Script1850GRC ans rename it Script
3) Do your change in this text file: by example for Ottomans, search for
SelectFaction = $TUR
MapOffSets = -33990;-4500
ScenVictoryCond = str1850ScenarioVC_TUR
SetCapital = $Constantinople;10
SetMoraleLevels = 75;135
ChangeFacMorale = 85
ChangeRailPool = 0
ChangeRiverPool = 60
SetTRSAssetsInfos = $uni_CMN_Sup_Rail;$uni_CMN_Sup_Riv;$uni_CMN_Sup_Su p1;$uni_CMN_Sup_Mch
SetFacUnselect = 1
AddRulerToFac = $ruler_TUR_Abdulmecid;0
Change the Bold line to SetFacUnselect = 0
4) Launch PON and press F9 key
5) Once the game is loaded, save the game.
6) Quit PON. You have created a new GC scenario with Ottoman playable by player. Now I don't know if the loading screen can support more than 8 Nations...
but it didn't work with me, I think you're gonna need more professional help

Kensai could help you I guess.