Can a Pokeball be used for things other than storing pokemon?
Like a "Hoi-Poi Capsules" from Dragon Ball or even to imprison humans?
They were made for storing pokémon, only. It can be seen, that some trainers play with them (at least in the games, there are jugglers and some psychics who make the balls float in the air). It can happen, that there are different things than a Pokémon in it, like when Ash found a riceball (?) in his Pokéball after the tries to catch a Mankey, which throws the riceball at the Pokéball, but besides from that, Pokéballs aren't used for anything else than catching and storing Pokémon.
In the anime, it was also shown that a Pokéball can't catch trainers, the beam that would transform the Pokémon to energy (to store it in the ball) just doesn't have any effect.
In the games, however, items which can be found in the world are shown as a red Pokéball. Whether it's for the sake of simplicity or whether they are really stored in Pokéballs is unclear (but you don't get a Pokéball with every item you find).