With the Desikan-Killiany atlas selected, select the scouts you want to copy and click on the Save button.
In the "User scouts" atlas, click on the Load button, select the file format "Brainstorm scouts", and select the file you just saved.
You can do the same with the menu "Scout > Export to Matlab" and "Scout > Import from Matlab", this would avoid saving a file to the hard drive.
Note that this precentral ROI is probably too large for using it as a scout for functional analysis. Test this carefully with your data before using it automatically with all your subejcts, and compare this approach against a smaller ROI manually defined based on the source maps.