You want to make the MIDI longer in Sibelius? I'm going to assume you want to sing more verses, so you will just be repeating certain parts. I don't know Sibelius, but I do use Noteworthy Composer. The two programs might work the same way.

First, right click on the MIDI on a website and save the MIDI to My Documents. Make a note of the MIDI's name as you do this. (Some MIDI's have very strange names.)

Open Sibelius. Tell it File- Open - My Documents - name you noticed. Then you might have to hit Enter for several boxes as Sibelius opens the MIDI.

When the file is open on your screen, highlight the parts you want to play again, hit Ctrl+C, then put the cursor at the place you want to music to play again, and hit Ctrl+V. The music will play again. (If you have a Mac computer, just do whatever you do to copy and paste.)

You have to do this for every staff. It can get tricky, and it helps to put commas to mark the measures where you start and stop, or else the music doesn't harmonize anymore.

After every Ctrl+V, go to the end of the MIDI and make sure the new line has the same number of measures as the melody. If it sticks out or falls short, a mistake was made. Undo the paste right away and try again. This is very important, especially in a fancy, complicated song.