Hi MeFree, welcome to the forums! The DAW plugin version of Band-in-a-Box does need to re-generate every time you load a new song project file into it. But if you're happy with the exact arrangement you had made in the main Band-in-a-Box program, you can export the files as audio files (or MIDI files) to your computer, then re-import them into your GarageBand tracks.

We have a tutorial here for exporting audio files from Band-in-a-Box for Mac: https://youtu.be/wRcqRzT0QmY

To export MIDI tracks, it is the save button to the left of the save audio button that says ".MID". Please note that only MIDI tracks (yellow or blue in the mixer) can be exported as a MIDI file. RealTracks (green) can only export as an audio file.

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Best,
Chantelle