Declarative business process models that are centered around artifacts, which represent key business entities, have proven useful to specify knowledge-intensive processes. Currently, declarative artifact-centric process models need to be designed from scratch, even though existing model fragments could be reused to gain efficiency in designing and maintaining such models. To address this problem, this paper proposes an approach for composing model fragments, abstracted into features, into fully specified declarative artifact-centric process models. We use Guard-Stage-Milestone (GSM) schemas as modeling language and let each feature denote a GSM schema fragment. The approach supports feature composition at different levels of granularity. Correctness criteria are defined that guarantee that valid GSM schemas are derived. The approach is evaluated by applying it to an industrial process. Using the approach, declarative artifact-centric process models can be composed from existing model fragments in an efficient and correct way.