This is a method you can use until this feature is implemented:
Options > Preference > Allow Multiple Instances of Band-in-a-Box to be Running

In the top BB we have our song, we just would like a different playing in bar 5-8.
Create a copy of your current song > my song (multiriffs).SGU
In your song top BB
We freeze the track we need the new section/s for
In the track menu > Tracks Actions > Save track as Performance file (wma/wav)
Select No to save as wav
Now Audio menu > Move Performance Track to Audio
Mute the original track
Select ACW or Audio Edit icon
In the Audio Edit select Snap
Highlight the section you need
Right click Cut (you can take it off of Snap and fine trim any notes)

In your bottom BB open the > my song (multiriffs).SGU
In the track menu select Select RealTracks > Generate MultiRiffs...
Select the same instrument
Generate for whole song
Make sure you have the mixer levels for all the riff tracks the same as that
instrument in the top BB
Solo each track one at a time and double click on the bar where you want to
play from, you can also highlight those bars and click on the loop icon in
transport and loop play that section while soloing different tracks.
When you find the bars you like highlight them
Now drag that track to WAV drop
When green drag it to the top BB Audio Edit track's same bar location (with Snap enabled)
In the Overwrite drop down select Merge (as the exported audio has an empty beat or so on the end)
It should all be the same volume level though you can select that bit and adjust with the "Edit ▼" Amplify +-
Do any other section then Audio menu > Move Audio to Performance Track

If you want the RealChart you can drag the bottom BB Riff track to MID drop
Import that midi (all channels) from the folder you exported it to to the Melody or Soloist track of the top BB
(with 714 you can now drag n drop midi directly into BB from the BB Plugin, file or another DAW.
You can then select the midi notes on that channel, cut and copy to another midi track.)
Now in the Piano Roll of the original frozen instrument delete the notes from that section
and copy the notes from that same section in the imported midi and paste them in your original frozen instrument track.
When you move the Audio back as a Performance to the original realtrack it will keep the RealChart notation/midi.

You can also in the top BB mute the Audio track
Drag the Master to WAV drop
Drag the green WAV into the Audio track of the bottom BB this will allow you to solo both the Audio track and a MultiRiff track to hear the riff with the rest of the tracks

To do it with 1/2 bar or beats you can move the Riff track to the Audio track as described above and highlight the bars and beats, right click copy now just paste that in the top BB Audio track by "Edit ▼" (in Audio Edit) > Paste (Mix).

You can also do this using the Biab Plugin in BB but at the moment the sync from BB to the Biab Plugin is not working correctly and needs fixing.
I will do a tutorial for that when it's fixed.
See Full Screen Track to Audio Edit Copy
See Full Screen Audio Edit Bar beat Copy

ReaTrak Integrate Biab & Plugin in Reaper
Free Biab & BiabVST Chord Picker Tool