Thank you for your post.
Astah does not have an ability to import JAR files we're afraid, but can import .java files. There are several ways to do so.
(1) Using Astah's built-in feature
Go to [Tools] - [Java] - [Import Java] from main menu and select a package or .java files.
(2) Using [Easy Code Reverse] Plug-in
Installing this plug-in (free) enables you to import .java files just by drag/drop a java file onto Astah or the URL of source code repository!
Easy Code Reverse Plug-in
At last, once you import all the data you want in Astah's list, you can save it as your template file and use if as a default model.
TIPS: How to customize a “default file” that opens when creating a new file
Hope this helps.