Sente can automatically import data from a Papers 2 or Papers 3 library.
For most users, if you have a Papers2 or Papers3 library on your computer, when you use the File > New Library, your library will be listed as an option in the dialog that appears. If you see your Papers library in that list, just select it and Sente will automatically import all of the references and attachments into your new Sente library. Your Papers library, including PDFs, will not be modified in any way.
If for some reason your Papers library is not listed in the New Library dialog, you can use the File > Import Reference Data command and select the library manually in the File Chooser window that appears.
For additional information on this topic, please see the chapter entitled Migrating Data from Other Applications in the Sente User Manual.