Rosegarden and LilyPond can be used together, which can greatly enhace your productivity. LilyPond will output a MIDI-format representation of your score if you include a "midi" section in the "score" section. It should look something like this:

\score
{
   ...
   \midi { }
}

This MIDI file can then be imported into Rosegarden by selecting from the menu 'File > Import > Import MIDI File'. Unfortunately, this will erase the current session, so it can be done only once.

It is also possible to export MIDI tracks to LilyPond format, which can then be edited further or simply turned into a score. To do this, from the menu select 'File > Export > Export LilyPond File'. After you select a location to save the file, Rosegarden allows you to control some score settings while exporting. After exporting a LilyPond file, you should inspect it with Frescobaldi before relying on it to be correct - computers sometimes make mistakes!