Yes you most certainly can!
Tap the Import Files button at the right of the main app screen to see some instructions for adding files to the app. (Screenshot included).
For MIDI files, you'll need to download the MIDI to your iOS device, then open up the Files app. You can download files on iOS from the web using Safari, or from Messages or Mail.
You can open the Files app from inside the SeeMusic app, by tapping the Import Files button, then tapping "Open Files app"
From the Files app, tap the file, tap "Share", tap "More...", then tap "Open in SeeMusic"
I hope this helps?