With the emergence of Cloud Computing and maturity of Service Oriented Architecture (SOA), the Software-as-a-Service (SaaS) delivery model has gained popularity, due to advantages such as lower startup cost and reduced time to market. A SaaS vendor owns and takes the responsibility of maintaining a single application for multiple clients/tenants who may have similar but also varying requirements. Business process modeling (BPM) approaches can be used to package service offerings to meet these varying requirements on a shared basis. However the customizations in those business processes can be challenging. In this paper we discuss the challenges arising from single-instance multi-tenancy, and present our approach to defining customizable business processes in SaaS applications to address those challenges.