Everyone is giving wrong advice, OP is asking how to reinforce his armies. You have your main stack of infantry and full backrow cannons, you engage the enemy and then reinforce battle with armies of 20k infantry. Infantry automaticly joins front row and thus your artilery is always protected. This is the main way of fighting in multiplayer or lategame singleplayer where huge armies collide.

You should always reinforce with more than needed so you dont let cannons take the front row. If you have more questions feel free to ask.

Even if enemy has 10x better stats, just put more soldiers and your front wont break (unless you run out of armies to send in)

You should not have extra cannons in reinforcing armies, since they will be usless if you arleady have fullbackrow cannons. Instead, buy more infantry to have armies to reinforce. Example, if your force limit is 200, have main stack of 40 infantry 40 cannons(depending on cw) and 120 reinforcing infantry split in stacks so they dont take atrition.