Current problems in Business Process Management consist of terminology mismatches and unstructured and isolated knowledge representation in process models. Semantic Business Process Management aims at overcoming many of those weaknesses of Business Processes Management through the use of explicit semantic descriptions of process artifacts. However, this vision has a prerequisite: semantic annotations need to be added to the process models. In this paper, we present an approach that allows flexibly annotating semantics in a user-friendly way, by exposing ontological knowledge to the business user in appropriate forms and by employing matchmaking and filtering techniques to display options with high relevance only. By adding semantic information the precision of process models increases, ultimately supporting Web Service discovery and composition. As a proof-of-concept, the work has been implemented prototypically in a process modeling tool.