If you damage an Ogre enough, there is a chance it might convert to you. This only works in Bal Rattha and in a few Orc quests.
What human units would you recommend to farm dragons?
Human Halberdiers, Elven Sentries and Orc Impalers all have bonuses against Dragons. The Dragon Slayer hero is, of course, a good option as well.
Try targeting a Dragon with Archers, distracting it with Cavaliers/Mounted Knights running past and damaging it with Halberdiers. Have some healing units like Knights or Heroes nearby as well to keep you from losing too many units.