It's a two step process:
1) Use our Dolet for Sibelius plug-in to export a MusicXML file from Sibelius
2) Use our Dolet for Finale plug-in to import that MusicXML file into Finale.
Dolet for Sibelius is available for Sibelius 2.1 and later. Dolet for Finale is available for Finale 2000 and later on Windows and Finale 2004 and later on Macintosh OS X. You can purchase the plug-ins at: