A few people have asked if it is possible to create a SharePoint Designer workflow variable that includes references to other variables. Indeed, it is possible.
The Build Dynamic String action in the Workflow Designer allows you to define any combination of static text, list field data, and workflow data in a single variable. When you click dynamic string for this action, you can look up values as needed in the String Builder dialog box, as shown here when creating a meeting request message.
To look up the value of another workflow variable, change the Source list value to Workflow Data and select the appropriate item from the Field list. In this example, the workflow already has a MeetingRequestDate variable that will be used in the dynamic string.
After you define the string value, click Variable: variable and then click Create a new variable to provide a new variable name.
It should be noted that these dynamic strings cannot be referenced when creating a task with the Assign a To-do Item action. Lookups are not allowed in the Name or Description fields of the Custom Task Wizard. However, you can reference dynamic strings when using the Send an Email action. Lookups can be performed in the To, CC, Subject, and message body fields of the Define E-mail Message dialog box.