You could simply selectively import the files to a library, but I guess you want preserve something that is stored only in the database (e.g. statistics and/or playlists).
You can merge file metadata between two libraries by using an MPL playlist. Load the files that you want to import to another library to a new playlist and export this playlist in the MPL format (File > Export Playlist...). Then load the receiving library and import the MPL playlist (File > Import Playlist...). It will import the files and all previous metadata -- except the "Date Imported" field which will contain the date of this new import. However, you can preserve also the original import dates by importing the same MPL file once again (use the File > Import Playlist... menu entry). Since the files are now already imported the "Date Imported" values will be updated from the MPL contents.
To transfer any other static playlists you must export and import them separately (you can use the simple m3u playlist format for that). If you want to transfer smartlists you must do it "manually". You may want to use the library that contains most of your smartlists as the destination library and recreate the possibly missing smartlists by copy-pasting the rules (you can use Notepad or some other external program for storing the rules).