I personally don't have experience trying, but a good friend of mine added one to her team without too much trouble. Really it all comes down to the individuals involved. Some Pokémon will join your team without even needing to be battled, simply because they want to. My Gothitelle, Alice, followed me around for two days before I got him a proper Pokéball.

[The screen tips a little. In the background a Gothitelle looks up from a magazine, blinks once, and then goes back to reading.]