Staking is an essential component in Proof-of-Stake (PoS) based blockchain systems. While a host of PoS blockchains have staking schemes in place, the implementations of those mechanisms are highly customized to meet the needs of specific blockchains and vary in terms of the offered functionalities. In this paper, we present EMS, an extensible and modular staking architecture for PoS systems. EMS specifies a generic and modular staking implementation framework by applying a novel bucket-based data structure across different system components. In particular, EMS is able to accommodate a variety of design requirements for staking in PoS systems by manipulating the optional fields in the bucket-based data structure, thereby providing great flexibility and extensibility. Our instantiation of EMS on the IoTeX blockchain further demonstrates its viability and effectiveness in practice.