It's a hard problem to solve, which doesn't have any fool-proof solutions (yet?).
Imagine, someone creates a centralized Twitter and a decentralized Twitter. How can you connect them two and being able to trust any data you receive 3rd-hand which is supposed to originate from the centralized version? The only information you can really trust, is the one you get from the centralized version, as it was built with that principle in mind, that you get it first-hand.
Similar problem with hooking together two monetary systems, one which is centralized and one which is decentralized. How could you include a centralized system within the decentralized one without making trade-offs that could potentially ruin the whole integration?
There are attempts at it, but as the problem is generally hard to solve (cryptocurrencies or tweets or any other data structure), it's hard to solve with cryptocurrencies as well.
