Since Rules are a component of a DDF, it is not possible to import Rules alone. However, it is possible to import a DDF from one document to another document. This imports all of the components of a DDF, including Rules. 

To import a DDF from one document (Document #1) to another document (Document #2), follow these steps:

1. Close the document (Document #1) that has the DDF you wish to import to another document.
2. Open the document to which you wish to import the DDF (Document #2).
3. Select DesignMerge > Setup > Data Source Definitions...
   
4. Click the Import DDF button.
5. Navigate to and select the document (Document #1) that has the settings you wish to import.
6. You have now added (or replaced) a single DDF in your current DDF Set.

Document #2 will now have the same DDF settings as Document #1, including the list of Fields, Variable Links, and Rules. 

Next, if the data file for Document #2 has a different list of Fields or if the Fields are in a different order, run Setup and when prompted, click the Remap Fields button, and follow the prompts. For more details about the Remap Fields feature, please see the following Help Desk Knowledgebase Article: How to add a new field, or reorder existing fields, in your DesignMerge setup

Then, if you edited the list of Variable Links after remapping Fields, also review each Rule in the Rules dialog window to confirm the Rules are pointing to valid Variable Links.