if you select the client (top level) and then deselect the drives that you don't want to back up then anything new will be included - the tick will change colour from bright red to faded red.
E.g. in the backup spec it will look like this if I want to include all new data drives but exclude C:\ drive
HOST "testserver.domain.com" testserver.domain.com
{
-trees
"\"
-exclude
"C:"
}