I think to let partisians work as a real division is not good, because in a strategy game, you can't simulate a tactic battle. So the best way to simulate the effect of partisians is that if an occupied area is not defended, or defended with not enough army, infrstructure, supply... of that area will be destroyed, and occupation troop will also lose time to time because of partisians.
Every occupied area should be defened by some amount of occupation troop. The amount of solders is determinated by the infrstructrue, population, terrian, and etc...
And, I think another factor should be included in partisians. that is a country can call on its people to join partisians to fight the enemy, the investment of this will deduce the effect of partisian attack too. What's more, the time that an area is occpuied is also an important factor when calculating the effect of partisians, because an old occupied area is often controled better.
So in this way, occupation army should be out in those important area, such as big industry center, resource producting area, important arterial road to the front, and in those area that are not important, few or no occupation army is used. I think in this way, the effect of partisian can be represented better. At least in pacific war, especially in China, this is the truth, Japanese have to put many many occupation army in areas they thought useful, but in other areas, they seldom came.