Historically, the French started with what in game terms would be three addidional army headquarters. Murat led the Reserve Cavalry, initally in direct command of 1st and 2ns Res Cav, with infantry forces added later. Eugene led the Army of Italy with IV, VI and III Cav Corps. Prince Jerome led the 2nd Reserve army with V, VII, VIII and IV Cav Corps. Army designations, attachments and leaders changed as the campaign progressed.
As the Russians have four army HQs, why shouldn't the French?
Leaders
Level 3 (Marshal or equivalent) models are provided in the models database, so they would have to be called as Davout3, etc. Seniority may need to be adjusted in the model files, so Berthier does not mind remaining as chief clerk. Some additional level 2 leaders can be added; Mouton (Count Lobau) and Vandamme coming immediatley to mind.
Army GHQs
Three would have to be added. What is the best way to add them to the Setup?
I will try this using the JSGME mod enabler, which I hope is supported by NCP.