Kakuzu's way of getting all the five chakra elements
This is just the theory, but I always assumed that since the blood vessels and the chakra network are intricately related, if you take one the other also follows.

Thus when Kakuzu takes the heart of the shinobi he also takes the chakra vessels tied in and around the heart. Thus he would push his own chakra into the heart while his unique body mimics the nature of the chakra vessels of the original body.

Thus he builds his own chakra, uses the handsigns to concentrate/accumulate it, but when nature transforming the chakra he passes it through the heart which acts like a processor to change the chakra nature to the type needed.