The Distribution List is saved as a file called Application.dat which is in standard xml format. You should be able to open this in Visual Studio (or other editing tool) and copy an existing list and then add to it (using an alternative name). Then when you re-open the tools UI you should see the new distribution list in the drop down.
The file should be found here:
C:\\Users\\<username>\\AppData\\Roaming\\Red Gate\\SQL Multi Script
If you find that you need more help, either post back here or email firstname.lastname@example.org
quoting the ticket number F0060634 in the subject.