I remember from somewhere that Athearn BB dummy locos are similar to the powered units but with no motor or drivetrain. Am I correct? Can they be powered by adding a motor and gears?
Yes a Athearn dummy can be powered…But…By the time you buy the motor,drive shafts and gears you would be way ahead to just buy the power unit.
yes
The cheap way to power an Athearn Dummy is to haunt the train swaps and garage sales until you find a powered frame which will attach to your shell. Putting power in a bare frame can quickly equal or exceed the cost of buying a powered unit. Sometimes, your well stocked LHS specializing in trains is willing to swap shells in order to make a sale. You can go to e-hobbies to check out component parts like powered trucks, 23t gears, 16t gears, worm gears, geared wheel sets, motors, shafts, universals and anything else I may have forgot.
Thanks all 
At the moment I have the chance to get a dummy AC4400 REALLY cheap, And I would use it as a dummy until I have the need to power it (which won’t be too soon)