Depends on why it's ineligible. If it's under 96 Kpbs, try right-clicking on the file and choosing Create AAC Version. (If you don't see this option, go to iTunes>Preferences, click "Import Settings..." then choose "Import Using: AAC Encoder" and "Setting: iTunes Plus".) Then rescan your library to see if this version will add to iTunes Match. Or, after changing your import setting, just re-import the song to your iTunes library.
If the song is over 200 MB you would have to reduce it's size to make it eligible.