Don't know if this is still relevant, but the first thing I do when opening a new document is Document Options->Load Library and add the 'Chord Suffix Expanded Times' library. It seems to have ever...
Use the Select button at the bottom of the "Chord Definition" dialog to choose the suffix. The first suffix (no. 1) is the triangle for major 7th. To add your own, use the Duplicate button in the "...