SibMEI is an open source plug-in that makes it easy to export MEI from the Sibelius notation editor. In addition to facilitating the creation of MEI-formatted data without the burden of writing XML by hand, it also makes it possible to convert MusicXML files to MEI.
The plug-in can be downloaded from https://github.com/music-encoding/sibmei/releases. General installation instructions for Sibelius plug-ins can be found at http://www.sibelius.com/download/plugins/index.html?help=install. Once installed, an Export to MEI option will appear in the plug-in list in Sibelius.