If you have multiple .txt or .onsong files, you can actually copy and paste them into your OpenSong/Songs/ folder (or subfolder). When you start the app, it will recognise them as not being OpenSong format and will try to convert them in the background. Let it boot up, finish the indexing (it may index the songs twice - once before conversion and again afterwards). If it doesn't update, restart the app and you should be good to go.
If you aren't sure of how to manually paste the songs in, you can make a zip archive from whereever your txt songs are and rename the .zip extension to .osb. The app will treat it as an OpenSongApp backup and import the songs.
Beware that any existing songs on your device with the same filename (without .txt) will be overwritten when the songs are converted, so I'd recommend pasting them into a subfolder first. e.g. OpenSong/imported/. If you go down the route of making a fake .osb file, paste the .txt. files into a folder named imported, then zip the folder. That way, when importing the backup, the app will keep the imported folder structure.
I'll have a think about adding the option for multiple file import in the future though.