Is it possible within the rules as written to take a unit or units from another army? the first thing that comes to mind is dwarves to get war machines to give long range shooting of the cannon
There used to be a Dogs of War choice you could take in place of a rare choice, if I remember correctly was back around 5th edition. As far as I know the only way to take anything outside your Army book is using the Storm of Magic or Monstrous Arcana rules.
Sorry man, we're the no fucn allowed faction. You want cannons, go play Empire or Dwarves. We're never gonna get anything like that.