In Taverna 2.x, you can add a workflow as a nested workflow by selecting Insert -> Nested Workflowfrom the top menu or right-clicking on the white space in the Workflow Diagram. Alternatively, you can merge a workflow with the current one by selecting Merge workflow from the Insert menu. More information is available in the nested workflows section of the user manual.
We recommend that you add a workflow as a nested workflow. This preserves the modularity and makes reuse and repurposing much easier. Credit and attribution of the original author are also clearer when you add another workflow this way.