I must admit I'm not terribly fond of some of the BIAB intro's the program generates.
Decided to see if I could import a keyboard intro ( saved as a midifile ) into a BIAB song instead.
I tried it with one of my psr styles & omb.
I loaded the style into the Arranger Window of OMB. Decided the key I wanted ( Cmaj) Typed in the Intro & the Chord & saved as midifile.
Next I created( edited) a generic 4/4 biab blank style ie I removed all the style parts except for one & altered the velocity for that part down to 1. BIAB appears to need one style part , even if it's only one note & not audible, without at least one note the style can't be saved .

I imported my midifile into a new biab song. Chose my "blank style" as default & enabled the style to play. BIAB automatically put in the chord changes for the intro. At the end of the Intro I used the F5 button to import a BIAB style.

The midifile intro actually gets loaded into the Melody section of BIAB. There's virtually a 16 track sequencer where you can edit the intro. Just make sure you have the "track type" set to 16 channel.

Also noticed I was best off staying away from the midi channels that are used by BIAB ie I used the default channel 9-16 used by psr's Mainly because there could be a conflict betwen BIAB style tracks & midifile tracks.

There are all sorts of editing options available for your midifile intro including a velocity off set just in case the midifile is too loud against the BIAB style.
Even has list editing.

The only thing I haven't been able to do is use it with a style where "real drums" are enabled. I think maybe that only one style per song is available when real drums are involved.

best wishes
best wishes

Korg PA4x, Korg PA3X, PSR S950, Band in a Box 2016
V Arranger/Ketron SD2