There is no dedicated menu to add back a flat reference channel before computing the average reference.
This is usually a concern when expecting the possibility to revert the data back to the original reference.
Do you need this for another purpose?
I guess there is one option to do this from the Brainstorm interface:
- Create a custom montage with all the channels being equal to themselves (eg. "Cz : Cz"), and with an extra line that multiplies any channel by zero ("REF: 0*Cz").
- Import the entire continuous file as a single epoch
- Make a full backup of your database, as the following step might damage things
- Create a new dataset including the extra channel with the process:
Standardize > Apply montage - Right-click on the file with the extra flat channel > Review as raw