When creating a sequence diagram consider creating the class somewhere first. Then drag the class from your project browser onto your sequence diagram to create an object. After having done this you can add operations to the class by adding messages on your sequence diagram.
If you then want to create a separate set of design classes, based on your analysis class model, you can copy and paste from one of your analysis class diagrams to a design class diagram, then select all, right-click, and choose Convert Linked Element to Local Copy.
The longer answer is to consider re-evaluating the analysis and design process you are using. I humbly recommend Craig Larman’s, Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process (2nd Edition). In this book he presents a trimmed back version of what the full-blown RUP offers, including not doing a pre-design, yet it is design, "analysis model". Instead, the artifacts he takes you through go from use cases, domain model, system sequence diagram, operation contracts, collaboration diagrams (design), and a design class model.
Best wishes,
James
