I have never seen the option to transpose a section of a song,
But I don't recall ever having a need. I would just write in the chords of the new key.

However I just did a workaround that may or may not work for you.

1. I started a NEW song in BIAB. 12 bar blues in C
2. Saved it on desktop as C blues.
3. Started another NEW song in BIAB. 12 bar blues in F
4. Saved it on desktop as F blues.
5. -----same process as #1 and #2 but key of G----

Then opened F blues, copied 12 bars, opened C blues
and pasted 12 bars of F blues in measures 13-24 of C blues.

Then opened G blues, copied 12 bars, opened C blues and pasted 12 bars of G blues in measures 25-36 of C blues.

Then stayed in the opened C blues, copied the first 12 measures and pasted them in measures 37-48.

It sounds like complicated, workaround but in reality took less time than to describe it here.

good luck