There is a scanner app called Scanner Pro by Readdle that would work great, you can then directly import the content from the app into OnSong. I believe it's $3.99 or something on the App Store.
Or if you go into the Notes app > iCloud > Notes > Select a new note > Tap the "+" Sign that in a circle > Scan Documents > Take a picture > Adjust how you would like the perimeter > You can then either retake the picture or keep scan > You can then either keep scanning or Save. > Tap the scan > Tap the Share Icon (square icon with arrow sticking out the top) > Save to Dropbox or the like. Notes. Will not let you save directly to OnSong. Once saved in Dropbox, you can import them into OnSong.