As you know, to enter a sub-title you press + and double click in the new title field.
When entering text you can also use the “enter” or new-line key to go to the next line and enter text.
I did not try at length but many lines can be added in this way.
Also using the T button for a multi-line subtitle will enable the alignment field for left, right en center alignment, which obviously is only useful when there are more than one line.
Note that some of this functionality became available in PD17.
Let me know if this works for you.
Regards
Warry