Currently the Java import supports only the reverse engineering of a static model (class diagrams). However, method bodies are already also added to the nodel, but just to complete the code generation result. But these lines of code contain valuable information about which sequences are allowed. These sequences should be made visible in sequence diagrams, hence using the imported methods in the model is desirable.
Alpha. Join the dev mailing list of the ArgoUML project and discuss this proposal!
However, a working initial version is already integrated in ArgoUML since version 0.24! Just select an operation which contains a method in Java notation (these will be created during full Java source import), right click on it in the explorer pane, and click on the "Reverse Engineer Sequence Diagram..." menu item. A dialog showing all calls in that method is presented. Select all and look how the sequence diagram is filled with content (not all calls are correctly processed, but it's possible to have calls to other classifier roles).