Blockchain is an emerging technology with the potential to resolve auditing issues. Implementing a new blockchain-related feature implies moving to a platform with another database or duplicating its parts in a blockchain system. Both ways are difficult to migrate and maintain. The alternative is to implement blockchain features within the existing database, including consensus mechanisms and specific data structures for audit needs. The paper describes and evaluates a database extension with blockchain-related structures, leaving consensus beyond the scope. We use an account-based prototype of cryptocurrency as a model example. The proposed extension allows provably checking transaction content and user balance without a full database lookup. The numerical experiments to study the overhead of the proposed extension are provided. CCS CONCEPTS• Information systems → Data management systems; • Computer systems organization → Peer-to-peer architectures.