You may also have access to that particular track in the Biab sgu file. Especially if you froze the track before you saved it in Biab.

IF.... you do have original track available in your Biab song, and, the riff is in that audio file, export that file and then import that track into RB.

IN RB.... The original track containing that riff can be extended 16 bars and the riff remain as long as the section you need to insert the 16 bars does not involve the particular bar or bars that contain the riff. In RB, you can cut an audio track, insert bars and generate just the 16 new bars and they will merge into the existing audio between the cut you placed. You will have to place the chords into the new 16 bars. If you are using pre-recorded audio, you will not hear any of that audio in the 16 bars, just the RB generated tracks.

If the original track containing the riff and the riff is in the new 16 bar section, you can possibly cut and paste just the riff and insert it into the correct bar. Success here depends on the particular song.

BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.