I am only an occasional user of Roundcube, but I also could not discover a way to do this. You may need to change your workflow to achieve the same results or use a different tool, such as a mail client.
There is also the option to create a second server that is exclusively for a separate Roundcube install. This would open up the option to install more plugins or otherwise customize Roundcube without fighting against the MiaB scripts. I’ve never done this, but I have looked at it and it seems viable.